Why did Hitler attack the Soviet Union when he was still busy fighting the United Kingdom?

Because Hitler was crazy. He was not normal by any standards.

Had he anticipated that US will join Britain and Russia against Germany, and that US will send practically unlimited military help to Russia -- even then he'd invade Russia, but he was doomed the day he invaded. Although it took 4 long years for Hitler to fall.

To add on the answers that already exist, in the traditional strategic order that existed before nukes.

land under control = K* men for war = k2* resources for war

This was the foundation for the traditional military order.

Hitler's conquest of France and Poland, was exactly mirrored in a way by Stalins actions in Romania, Finland and Baltic. This was natural, and the aim was to give away no strategic gain.

But Germany was still very dependent on Soviet trade for crude and others such vital resources. Here lay the necessity in going after Russia. The ideological element wouldn't have mattered much if Hitler did not feel Russia had an effective 'kill switch' over German industry which he knew it did.

But one of the earliest German plans, I think, wanted Russia to be pushed over the Urals, and creating a new status quo around everything to the west of the Urals. This would have made Germany predominantly self sufficient, and was obviously a sound doctrine. The tipping of the balance, in my view, was the entry of the United States, and the fact there was an winning war of attrition that the allies could pursue, which they finally did.

Short Answer: Molotov–Ribbentrop Pact was always going to end badly. Staling hated Hitler and Hitler hated Stalin like no other leader in Europe. That Hitler struck first is probable not as big a mistake as it seems. Germany and the Soviet Union were always going to mix it up. Hitler's big mistake in attacking the Soviet Union was not advancing on Moscow in a timely manner and getting distracted. Most Historians split on whether the German General Staff would have been successful in Russia without Hitler's meddling. Hitler's delays in attacking Moscow meant the poorly outfitted German army had to fight through one of the coldest Russian Winters in History and the German army never recovered.

Longer Answer: Hitler was capable of great innovative ideas, but he was also capable of huge mistakes. Most of Hitlers generals believed invading Poland was a mistake because at the time Germany didn't have much left guarding the boarder with France. Then most of his generals thought invading France was a mistake, given WWI's experience and conventional wisdom being that the French Army was the strongest on the continent, not to mention Great Britains forces. In each of these instances Hitler had proven correct despite his generals objections. One of the great debates in WWII is whether left to their own devices, without hitlers meddling could the German General staff had been successful in Russia. It's unclear, but what is clear is without Hitler they likely wouldn't have attacked the Soviet Union in June of 1941, and with Hitler they couldn't be successful.

Hitler had gained popularity in the 1920's upon four foundational points.

(1) attacking the Treaty of Versailles (2) promoting Pan-Germanism, (3) anti-semitism (4) anti-communism

Hitler who was actually incredible poorly informed on many subjects which he liked to talk like an expert on, like internal American politics.. and like how influential Jewish citizens were in every country he wanted to invade. Hitler often equated anti-communism and anti-semitism as he believed communism was a jewish plot to control the world.(**) That Hitler hated communists more than the British came to news to nobody. Hitler and the Nazi's had fought street fights against communists in Germany before coming to power. He hated communism with a passion and stated as much in 1000 speeches and prominently in his own biography Mein Kamf ( my struggle). Hitler dreamed of purging Russia from Jews and Communists and capturing all that land for German use, probable in that order too.

So why did Stalin align himself with Germany. why did the Soviet Union agree to the Molotov–Ribbentrop Pact in August 1939. Because the Soviet Union was totally unprepared for war and was buying time. Stalin was trying to prepare as quickly as he could, as all of Europe was preparing. The Molotov-Ribbentrop Pact bought Russia two years, which was probable worth it for Stalin. Hitler betrayed Stalin attacking in June of 1941, because he saw an opportunity which probable didn't exist to take the Soviet Union out quickly before they could fully prepare. Which brings us back to colossal mistakes which Hitler was capable of. Yes it was a mistake to invade the Soviet Union because it caused Germany to be again be trapped in a two front war. The real fatal miscalculation was when Hitler paused and diverted forces from Army Group Centre to the south towards Kiev Ukraine. The delay stalled the attack on Moscow and forced ill-equipped(winter clothing) German troops to fight in one of the worst Russian winters ever recorded. Germany never recovered. The Wehrmacht was not able to sustain its pressure on Moscow, instead fighting, and eventually retreating, across all Russian teritory.

(*)Why was Moscow so important? I saw a previous comment about Napoleon took Moscow and it didn't help him avert a catastrophe in Russia's winter. The answer is Trains. Moscow was the central train hub for all of the Soviet Union. If Hitler had controled Moscow he would have controled all the train systems which Russia used to transport not only troops but all their military and civilian logistics. If Hitler had taken Moscow it would have been game over.

(**) Hitler also believed capitalism was a jewish plot to control the world so go figure.

The problem was that the attack on England had failed. This meant that there was no way to relieve the blockade on Germany. Hitler was bankrupt with no options other than attacking Russia. They were literally ripping up hand railings in Berlin to get scrap metal. Everyone was on rations which were growing more stringent by the month. The Soviet Union was the only attackable place with anywhere near the resources necessary to affect the economic situation in the Reich.

The ideal would have been to conquer Britain and take control of the Atlantic. That would have opened up trade and improved the situation, but with the English in firm control of the channel and Germany's navy far too weak to challenge England, a land war against the Soviet Union was only option left.

Do not put faith in anecdotes and people's words about their motives. When push comes to shove your wallet dictates your actions, and Hitler's wallet was empty.

To add to some already good answers.

It didn't matter hugely.

The most important aspect of having a war on two fronts was having to guard the Atlantic coast. For the most part this meant tying down the requisite number of armed forces to ensure that invasions or raids could not be conducted (or at least without huge risk).

Winston Churchill had made it clear that defeating the UK (a pretty daunting task as it stood) would not mean an end to conflict in the west. His famous speech in 1940 made it clear that the UK government would continue to fight on the island, even after losing land superiority, and even if entirely ejected from the British Isles, would continue resistance within the Empire. Partisan action could also be assumed in occupied territories. As such, a large number of German soldiers would be tied down for a long time in the west, even if the war on that front was "officially" concluded.

The other aspect to note was that it was considered to be a bad idea for Germany to fight a war on two fronts. Stalin did not believe that Hitler would ever invade the USSR while Britain remained a threat. To perhaps lend credence to this idea, the largest air raid against Britain was conducted in the run-up to Barbarossa (10th-11th May). Despite intelligence to the contrary, the USSR (and Stalin in particular) was caught napping when Germany launched an invasion on 22nd June. This had the effect of complete devastation of the Soviet forces in the west. However, as war with Germany had been considered ultimately quite a likely scenario, much of the soviet industry had already been repositioned behind the Urals, meaning that it would be virtually untouched throughout the war.

While fighting a war on two fronts eveutally allowed the Allied to land in France during Overlord, by that stage the war was already decided.

Germany already fought on two fronts. This is the first world war. And Hitler fought in this war. He was not an idiot.

Germany in the 1920s is an outcast country. The same outrage as the USSR. Germany and the USSR helped each other. But... The USSR is the single socialist country in the world. But the "problem" was that this single socialist country was developing rapidly. Industrialization, free medicine, free education, free land, autarky. This is a threat to the entire capitalist world. It is a flag, a symbol for all Communists in the world. Stalin said that "either we will become an industrial power or we will be destroyed." All the Bolsheviks understood that the USSR was the number one target. The question was: who, where and when will kill the first country of people's democracy. The choice was perfect: Germany. In one moment a friend turned into an enemy. Hitler - huge assassin. Hitler received Czechoslovakia. Hitler received Poland. France did not want to fight and actually gave up. Even the Poles defended the capital. The French applauded the German troops. When Hitler became strong, he attacked the USSR. Because Hitler is a hired killer. The USSR is a victim of a hired killer. Everything is very simple.

By the way, there is one more "strange" moment. "Idiot" Hitler did not mobilize the economy. If you are at war, you must mobilize the entire economy. I suppose the master just kept his dog on a leash. The owner was afraid that Hitler could get out of control. The owner put on a muzzle - he forbade the mobilization of the economy.

When the hired assassin missed, the customer organized his destruction.

p.s. In 1941, Hitler controlled more factories and engineers than Stalin. Mobilization + technology = thousands of Panzes, Stugs, Ganomags, aircrafts, artillery, divisions, corps, armies. The USSR could lose, but the German shepherd had a short leash.

I think the best approximation to a true answer is that he wanted to kill all Jews as quickly as possible. Britain had little Jews, while the western part of the USSR and Poland had the highest population of Jews in the world due to pale of settlement of the former Russian Empire which mandated Jews to live only in the western parts of the country.

In 1939 Gestapo made a research and concluded that the western parts of the USSR were a biological base of the Jewry.

Stalin was likely to attack anyway, and made statements in this direction.

Source: Soviet offensive plans controversy

Danilov and Heinz Magenheimer examined this plan and other documents in the early 1990s, which might indicate Soviet preparations for an attack, in the Austrian military journal (Österreichische Militärische Zeitschrift, nos. 5 and 6, 1991; no. 1, 1993; and no. 1, 1994). Both researchers came to the conclusion that Zhukov's plan of May 15, 1941 reflected Stalin's May 5, 1941 speech heralding the birth of the new offensive Red Army.

The reasons given in the other answers, while well-cited, are not necessarily the actual reasons why Germany attacked. The Nazi leadership probably wanted to frame the war as an aggressive conquest that would be successful, not a desperate pre-emptive strike to slow down a much stronger opponent.

In any case, both sides expected the war to happen. Stalin believed that the war between capitalist nations would create ideal conditions for a Soviet attack. Hitler also expected to fight Russia as stated in Mein Kampf. Given these assumptions held by both sides, the only logical outcome is that Germany would attack first, in 1941, when it had just conquered Western Europe and was at its strongest.

Because Finland was never a puppet state, it was one of the Axis countries. The Red Army invaded Finland in 1941. Secondly, in 1941 there were highs and lows between Nazi Germany and Soviet Russia. Thirdly, the Red Army came near the east Polish border. Fourthly, when Greece and Yugoslovia were invaded by the Third Reich, the Soviets supported Greece and Yugoslovia and aided them .

One possible cause, put forth by Viktor Suvorov, was that Germany became aware that Soviet Union was planning invasion of Germany as soon as Germany would exhaust most of its resources in the West. In his books "Ice-breaker" and "Mobilization" he provided some evidence to that, such as concentration of Soviet troops at the border, removal of mine fields along the border, production of primarily offensive weapons, propaganda slogans aimed at "liberating the workers worldwide", etc.

The direct reason for Germany to attack the Soviet Union was Soviet advanced preparations to invade ('liberate') central and western Europe. The Soviet Union had the most powerful army in the world at that time and began to concentrate the troops along the new Soviet-German border.

Effectively, Germans stopped Stalin's plan to establish communist rule in Europe. This view is not popular among historians, because Nazi Germany committed horrendous crimes against humanity and Soviet Union was a Western ally for some time.

Listen to Hitler's speech dated October 3, 1941.

First of all, I don't think the reason was primarily a political one. I think it was a decision from a military-strategical viewpoint.

In 1940-41 Germany/England weren't able/strong enough to deal each other the final blow. Both sides were in the curiously military situation (because of the might of the "Royal Navy" and the numerous/technically advantage of the continental German "Heer").

To march to Berlin, the UK needed a strong continental/economical power at her side (for example, the Soviet Union or USA). To land in or successfully besiege the UK, Germany needed to build a lot of naval/air vessels (bombers and U-boats), and for that enough resources.

With the elimination of the Soviet Union, Germany would have prevented a potential continental threat (alliance between UK and Soviet) and would have gained enough resources to even fight the sea powers UK/USA.

I think it primarily came down to Germany's dwindling oil supply. They were mostly running on a large stock pile that was running out. Invading the Soviet Union meant access to large oil fields which would have fueled the Nazis for a long time.

I think if they had got to the oil, the world would have been in for a hell of a showdown even with the Americans having the atomic bomb. Let's not forget that the Nazis had the knowledge of fission, had some of the greatest scientists of the time and controlled a plant that processed heavy water.

According to Van Manstein and Admiral Raeder who both advised Hitler to attack Great Britain immediately after crushing France in 6 weeks "they were amazed to observe that Hitler thought Great Britain was his Ally." So that ended any attempt at formally invading Great Britain...probably the only time Nazi Germany's staff was in a hurry to attack and Hitler wasn't actually.

As for Babarossa Hitler stated clearly that as it related to an invasion of Russia "we were peaking miltarily in 1941 yet Russia wouldn't be for at least another year." So you tell me what that means cuz I sure don't understand it.

Anywho David Glantz has written the best History by anyone concerning Generalplan Ost as he accessed the Russian Archives once the USSR fell...which showed on paper at least the Red Army to be overwhelmingly superior to the German Wehrmacht in June, 1941. Even Hitler said to Goebels "the World will hold its breath" when he kicked off the whole thing...so I think it's wrong to say he was delusional...at least initially.

One major economic reason to invade Russia was for slave labor however. The 3rd Reich had massive manpower shortages during both rearmament and in fighting Britain. Since Stalin had incongruosly moved up the totality of his manpower right to the Polish border Barbarossa in theory and in fact did solve the Wehrmacht labor shortage immediately.

In other words while the Army would suffer massive manpower shortages starting in 1942 the German economy would not.

This had devastating consequences for France, Britain and Russia once they began the operational struggle for launching actual offensives against the "3rd Reich."

None of them could have succeeded without the Americans...and the USA knew that.

Since Japan attacked the United States on December 7th, 1941 I think the more interesting question is why the USA thought it could win a War on two fronts actually...which the USA did in fact do.

That might explain Hitler's thinking when it came to a "two front War."

In short Hitler had a very low opinion of American "fighting power"...as he often did of his enemies.

It boils down to Sea Power, or rather, Germany and Russia's lack of it.

Germany and Russia are both Land powers. They tried to establish sea power, but their navies got destroyed. (Germany in WWI, Russia in the Russo-Japanese War).

As long as the British had free reign in the Mediterranean, Hitler must go east to seek sources of materials (oil, gas, metal).

Hitler also knew Russia is going for him, since they lost badly in the east and there's no power in middle Asia that they'd need to fight anyway.

So he attacked while the Soviet Union wasn't ready. It was a gamble that he ultimately lost, but he had no other choice.

Basically, he had no choice. He knew Stalin was preparing for war. The Ribentrop-Molotov Pact was just meant to give Soviet Russia more time.

You can read more on this in Victor Suvorov's books (i.e. "Icebreaker"). However, he is not a historian and many people regard this as a controversy.

Germany and Russia were never truly friends, and merely exchanged "agreements" every now and then. They have a history of mistrust. But more importantly it is a myth that it was a "bad idea" to invade Russia when they did. It was actually a very brilliant idea for the time. Don't forget Stalin was totally under Hitlers spell and did not even believe his own advisors Germany was planning to attack. I agree with others who state it was not executed quick enough. And it came down to a few small minor details that changed the out come. It could have just as easily gone in favor of Germany. I think if Germany could do it over again, they would do it again too, but perhaps only sooner :)

1. There was little action taking place between Britain and Germany at the time. And Germany just single handedly decided that they were at peace with Britain and the States.
   
2. There was very little oil to be found on the entire european continent, meaning germany desperately needed to capture an oil field somewhere. The choice was between going into the middle east, or going into Russia. Russia had been a long time foe and hitler had already declared it an enemy in his book "mein kampf". So by invading Russia Germany could potentially solve two problems at the same time, Russia and access to oil.
   
3. The soviet union had just been humiliated in its invasion of Finland making them appear very weak. And also Stalin had killed millions of Soviet citizens already, making them again appear weak.
   
4. Germany was very concerned about a war on two fronts and figured it would be better to have already dealt with the Soviets by the time the allies attack.
   
   And at last, this thing about the oil cannot be overemphasized. Without oil all wars are automatically lost and it was the main reason for why Germany lost. As late as april 1945 Germany still had massive amount of, for instance, U boats. But they just didn't have any oil to operate them so they were utterly useless.
   When they lost in Russia the rest of the war was just formality. That's why Hitler was so persistent on not retreating from Russia, giving up the Russian oil fields ment the fighter aircrafts, bombers, tanks, ships and u boats would soon all stop operating.
   The war was lost.
   
   

"Everyone knows that having to fight on two opposite fronts at once is bad..." It can be, for sure. Everyone looks back to 1812 and Napoleon rather than 1918. Germany fought a two-front war and effectively won in the East with the Treaty of Brest-Litovsk. The Germans also believed (somewhat correctly) that the resources gained from their occupied territories in the East had enabled them to keep fighting in the West. Hitler intended to apply and expand that "lesson" from the first war.

Now, to think of invading Russia as "opening a second front" is to overestimate the UK's status as a "front". It was a "front" insofar as Germany chose to make it a front. When they decided not to invade in 1941, it did not become a "front" again until June 6, 1944. Yes, bombers and all that, but those weren't really a factor until 1943, at least a year after which the war against Russia was supposed to have been won.

In one of the most stunning examples of misplaced over-optimism in history, Hitler blithely assumed the British were a broken force, unable to do him any meaningful harm from where he had them pent up in their little island. Remember, perhaps the singular most important pillar of Hitler’s ideology was the “drive to the East” to obtain “living space” (lebensraum) for the German master race. And that meant conquering first Poland and then Russia. As has been stated elsewhere by respected historians, the only two wars Hitler really wanted were the wars against Poland and Russia. When France and Britain went to war in the name of responding to Hitler’s aggression against Poland, Hitler regarded it as an irritating distraction from his true overriding passion to conquer all that good farmland and natural-resource rich land to the east. Hitler would never have invaded the Low Countries and France, he never would have tangled with the British Empire, if he could have avoided it. But by going to war over Poland, France and the U.K. forced Hitler to invade Western Europe even though he didn’t want to. Hitler felt enormously relieved that France was overrun so quickly and Britain reduced to such apparent impotence because it freed him to refocus on his real target, Russia. When Hitler invaded Russia while Britain was still alive behind him, it was a supreme example of ideology being given precedence over strategic common sense. (It makes one wonder how history would have gone if the French and British had acquiesced in letting Hitler take whatever he wanted as long as he was heading east.)

Germany failed to achieve air and naval superiority over the English Channel, which meant that they could not launch a land invasion (Operation Sea Lion). This left both sides in a stalemate. Hitler had been planning to invade the Soviet Union anyway, so with the British seemingly contained the Wehrmacht was free to launch its invasion of the Soviet Union. Hitler also thought that the Soviet Union could be quickly defeated and therefore did not worry about a two-front war.

If we look at WWII from the point of view of resources, we can somewhat justify Hitler's move of invading the Soviet Union even while being engaged in war with the United Kingdom.

Invading the Soviet Union was part of the Hunger Plan. Hitler was always demanding 'lesenbraum' or living space. In this context, greater living space meant bringing more land under agriculture. German economy was already reeling from the Treaty of Versailles and there was an acute shortage of food grains in the country. This meant it was difficult to feed both civilians and soldiers. Also, Hitler wanted to make Germany self-sufficient in terms of agricultural production. Germany had already witnessed what a disaster it could be when relying on trade for food from the naval blockade of WW1.

The question may arise why Germany turned on Russia even while fighting with the Great Britain at the same time. Well, the architects of the Hunger Plan, namely Herbert Backe, envisaged that:

The war can only be continued if the entire Wehrmacht is fed from Russia in the third year of the war.

This can provide the answer to why Germany was so desperate to execute operation Barbarossa (in 1941, 3 years after the war began, just like the plan predicted). They simply could not sustain the war effort otherwise.

Basically, Germany had a bare preponderance of strength against Britain. It sought to increase that preponderance by first conquering the Soviet Union, then dealing later with Britain, (and the United States). The idea was that Britain could be "contained" while Germany conquered the Soviet Union, and then beaten afterward by a Germany that was "reinforced" with the proceeds of a Soviet conquest.

The Soviet Union had key resources, Ukrainian wheat, minerals in the Donets basin, and Caucasus oil that could be used in a fight against Britain, and if necessary, the United States. Unlike Britain itself, all of these resources were within the grasp of Hitler's vastly superior land forces. And the Germans had the totally wrong notion that the Soviet Union could be defeat in a few months (unless they came to a "political" solution that would spare Russia, while Germany got the above-mentioned non-Russian territories).

Put another way, the idea was to "recycle" conquered Soviet resources against Britain (and the United States). The reverse was basically unachievable; while the British Islands had considerable manufacturing capability, the colonies, Commonwealth, and the U.S. itself would stop sending raw materials to Britain if it fell into Nazi hands, leaving a shell of a manufacturing power.

This strategy was supported by the so-called "Heartland Theary" of Britain's HJ Mackinder (and Germany's Karl Haushofer). That is, if Germany conquered eastern Europe (including Poland, the Baltics, Belarus, the Ukraine etc.) it would dominate Eurasia's Heartland,(which would also include Russia), giving it a head start on controlling "world island" (the Eurasian land mass).

As this Naval War College study on Barbarossa points out, Hitler was initiating plans for an invasion of Russia as early as July 21, 1940:

Whatever the thought process, on July 21, 1940 Hitler directed Field Marshal Walter von Brauchitsch (Commander-in-Chief of the Army) to develop a plan for the invasion of Soviet Russia to begin in the fall of that year. The staff was able to dissuade (sic) Hitler regarding that time frame, and then began to plan for an invasion which would take place during 1941.

These studies occurred simultaneous to the heady days of the Luftwaffe's highly successful attacks on British airfields (during the Battle of Britain) and the Kriegsmarine's first U-boat Happy Time, before the initial cracking of U-boat Enigma codes.

This was the direst period of the war for England, with completely unsupportable losses of aircraft, pilots, merchant marine and escorts occurring daily. U.S. isolationism was still strong, and Lend-Lease (March 1941) still far in the future. If Dolphin (Kriegsmarine's U-boat ENIGMA cipher) had not been cracked in early 1941 using captured German code books, allowing convoys to be routed around U-boat wolfpacks, a forced capitulation of the UK is quite conceivable:

Nothing frightened me during World War II except the U-boat peril. - Winston S. Churchill

Against this background, a decision to invade the USSR before its recuperation from the purges of 1938 could be completed and simply await an inevitable surrender of the UK seems almost sensible. Fortunately the code-breakers at Bletchley Park received the pinches they needed and the UK survived after all.

And from the Chapter 2 of Bryan Fugate's Strategy and Tactics on the Eastern Front, 1941:

Although the information gathered from these flights was not conclusive, it became obvious that in the event of war the Soviet threat to Rumania and the Wehrmacht's oil supply would become very great. The decision, then, to deal a heavy blow to the USSR before its potential threat could grow much beyond the existing level was one that found easy acceptance among German military leaders. They certainly were not motivated by abstract ideas of "living space" in the east, nor were they dedicated to the grander concept of a "Greater German Reich" stretching from western France to the Black Sea{61}. This is not to deny that the non-Nazi generals favored territorial expansion for Germany, but none of them are on record as having endorsed Hitler's most extreme proposals in this respect. They did believe, however, that after 1941 the relative strength of the USSR, economically, diplomatically, and militarily, could only increase, whereas Germany's could only decline as long as the war dragged on in the west, a war that eventually might well mean the involvement of the United States{62}. Halder himself said after the war that no nation should be denied the ultimate right to launch a preventive war if that is the only alternative left open to it{63}. The Russians, too, are not loath to admit that their country, already under a massive war-oriented economic program [92] inaugurated by Stalin in 1929, would have been in a much stronger position in 1943 than in 1941{64}. The Third Reich's best chance was in 1941, albeit a slender one.

OK, here is the real answer. The entire purpose of the War was to fight Russia. Russia was Hitlers main foe and ultimate objective from the start. He did not want to destroy the United Kingdom or United States per se, he wanted to build a Germany-centered Europe and trade with them (ironically kind of how it is today). Basically he wanted Germany to become what the English Empire was. He had to take care of the other European powers (France, Poland and the UK) first either through conquest or treaty specifically so he wouldn't be fighting a two-front war with Russia. Something he pretty much accomplished.

Thus he cuts out a deal with the Italians, Fins, Japanese, and Soviets (sneaky sneaky) and takes Poland, France, Norway and a large chunk of North Africa. He runs into trouble with England and has to jump ahead in his plan. Note that after the Battle of Britain, the UK and Germany were at a stand off. Neither could invade the other due to the logistical problems of crossing the channel with an invasion force. Germany figured the UK was no longer a threat by building the Atlantic Wall and moved its resources to the East. It is important to note that Hitler didn't think much of the United States at the time. He did not foresee the production capability of the country when he declared war.

Germany then turned to Russia (its "big" enemy) and launched the war Hitler dreamed of. The War to destroy the Soviet Union for Lebensraum.

The Soviet Union military had just been purged by Stalin and done very poorly in the Winter War. Germany had no reason to believe they would be a problem. A combination of larger population, harsh winter, and massive lend lease from the United States allowed the Soviet Union the ability to win four main battles. Battle of Moscow, Battle of Leningrad, Battle of Stalingrad and the Battle of Kursk. After these battles the majority of the German military machine was spent.

Most historians don't consider the two-front war to have started until the United States and United Kingdom broke the Atlantic Wall in France on D-day. Hitler knew that the invasion would come from the West eventually, but thought that he would have defeated the Soviets long before and be able to then focus on the Western defense. Obviously not the case, he fell into his worst nightmare and had to split his resources like in the first World War.

Fun fact: Hitler thought his biggest danger would be France (as it was in the first World War) and that the Soviet Union would fall like a "house of cards". Ironically it was the complete opposite.

Hitler turned his attention from Britain to the Soviet Union because he lost the Battle of Britain and because Nazi ideology left him no other choice.

In the summer of 1940, Hitler launched a massive air campaign to destroy the RAF and pave the way for Operation Sea Lion, the invasion of Britain. Hitler believed that to get past the Royal Navy he needed air superiority and so ordered a campaign of concentrated bombing to destroy the RAF. He almost succeeded, but Churchill calculated that if he bombed Berlin Hitler would be forced to respond in kind to preserve his reputation and credibility. Thus Hitler, not knowing just how close he had come to destroying the RAF and under pressure by his own military to retaliate, was finally forced to switch targets to London, effectively ending the campaign against the RAF.

The change of targets was damaging to the Luftwaffe in two ways. First, pressure was lifted from the RAF, which allowed it recover, and second, with the increased range demanded for bombing London, Luftwaffe bombers were often left without fighter cover and thus became easy targets for the RAF. On 15th of September, 1940, two massive Luftwaffe attacks were completely defeated by the RAF with 60 German aircraft shot down. After this defeat, Hitler had no option but to postpone the invasion of Britain indefinitely.

This, Hitler's first defeat, turned opinion in the United States. Up to this point many Americans had assumed that Britain was bound to fall, but after Hitler lost the Battle of Britain, Americans started believing that Britain would survive and the United States began supporting Britain much more aggressively. The help from America, the rapid rebuilding of the RAF, and the strength of the Royal Navy, made the invasion of Britain next to impossible.

With invasion of Britain no longer a possibility, Hitler turned his attention to the only other obvious venue for expansion: the Soviet Union. It must be noted that Hitler had promised in Mein Kampf (1925) to invade the Soviet Union, asserting that the German people needed living space (Lebensraum) and that this space lay to the East. Also Nazi racial ideology considered the Soviets subhuman (Untermenschen) ruled by Jews whom they despised. According to Nazi thinking, mere subhumans would be no match for the superior qualities of the German race and the Jews had to be eradicated everywhere. Thus, there was almost a religious mission to liberate land from Jews and subhumans and prepare it for the supposedly superior German race. Hitler had to invade Russia because this was the only way to deliver the ultimate goal of Nazi ideology: to establish Germans in their rightful place as the masters of the world.

Germany wasn't "still busy" fighting Great Britain at the time.

The conflict against the GB was never really likely to develop onto a full-scale battle as Germany lacked the basic resources to attack mainland GB.

Hitler had no landing craft, the "Rhine barges" that have often been mentioned a being possibly used as landing craft would have been hopeless in such a situation as they would have capsized in anything but the calmest of waters.

The Royal Navy was also far too strong for the Kriegsmarine to have afforded any lasting protection to such landing forces, while they may have managed to land on the beaches of southern England they would not have been able to maintain naval supremacy to allow reinforcements or supplies to arrive in the coming days.

Even had the Luftwaffe managed to subdue the Royal Air Force, a feat that they came fairly close to achieving, this would have had little effect on sea actions. It's been well documented how bad the Luftwaffe were at engaging warships, look at how little damage they did to British forces evacuating at Dunkirk for example. Dunkirk was a good trial run for the Luftwaffe to see how they would fair against the Royal Navy and yet they had extremely poor results against ships that were often sitting ducks, not moving but tied up to the harbour wall while the troops embarked.

Hitler's last desperate gamble in changing the focus of the attack from the Royal Air Force and onto the cities was a vain attempt to get Britain to surrender or at least sue for peace.

So Hitler knew he had to attack Russia, before Russia attacked Germany. Britain at this time was not a major problem to Germany, Hitler knew they couldn't mount any serious opposition other than a few air-raids and even those were limited to night attacks as the Luftwaffe could mount a good defence of mainland Europe in those days.

The Russian Army was in a terrible state at the start of the war due to Stalin's purges where many of his best commanders were executed or sent to the Gulags in Siberia for not being Communist enough, to paraphrase.

Their armed forces were generally badly trained and equipped too, while the T34 was available it was only in relatively small numbers and they had only a few modern aircraft and the ones they did have generally did not have trained pilots to fly them

Arguably Hitler should have attacked even sooner, but he got bogged down helping the Italians in the Balkans conflicts and so Barbarossa did not start until June 1941, a month or so earlier might have made a big difference to the outcome.

So, the reality is that the Germans weren't actually fighting on two fronts as the war with Great Britain was limited to the Luftwaffe providing air-defence of the Reich. Most of the troops deployed to defend the coastline were poor quality conscripts and even prisoners.

After Soviet performance in the Winter War in Finland, it was believed that the Soviet army could be rapidly and easily defeated. The Red Army had started to undergo significant reforms based on its experiences, as well as modernizing much of its equipment, particularly tanks. While Germany was unlikely to become significantly stronger over time, the Red Army would get better prepared, the longer the Germans waited

Unfortunately, we cannot ask Hitler about that and he didn't leave any written notices about his reasons. So every answer to that question is a speculation and I've seen a number of such speculations.

• The official Soviet version states an ideological war between fascism and communism that prompted Hitler to attack the Soviet Union without considering logic. A common "proof" are references to "Mein Kampf" where Hitler discusses the possibility of expanding the German Lebensraum to the east. This version is actually pure non-sense: there never was any such ideological war, with the fascist and communist ideologies being very similar Germany and USSR cooperated closely, especially after the Molotov-Ribbentrop Pact. The local conflicts between fascists and communists in Spain and Germany were rather due to the fact that both parties addressed mostly the same electorate. Also, by 1941 the Germans already occupied more territories than they could possibly process in the next decades.
• Then there is the version that Hitler and his generals got megalomaniac after all the victories. They knew perfectly well from WWI experience (not exactly ancient history at that point, all of them were there) that a war on two fronts is deadly. But they simply expected that the war against USSR would be over similarly fast as the war against France. This is a pretty ridiculous assumption: even occupying Moscow doesn't mean that the war is over (as Napoleon already discovered), the USSR offers plenty of room for the troops to retreat while the supply channels of the German army get too long. In fact, conquering USSR was obviously strategically impossible and Hitler could have been driven only by the assumption that the Soviet regime is too unstable and would immediately collapse (an assumption that he supposedly expressed but that turned out to be wrong as we know).
• In addition to the points above, Daniil Proektor brings up an additional point in his book "Aggression and Disaster": a defeat of the Soviet Union would have demoralized Great Britain that supposedly only continued fighting because it hoped to get help from Soviet Union and USA. I think that I've seen other historians mention that possibility as well.
• I'm not sure whether anybody considers this as a reason for the German attack on the USSR but once Germany went into war in 1939 it had to continue fighting and winning. Any lengthy period of peace would have made the German economical problems obvious and could have led to the collapse of the regime. The German external debt was already astronomical in 1939 and the war wasn't cheap. With the Battle of Britain lost, the victory in the war against Great Britain wouldn't happen any time soon - so Germany had to find somebody else to invade. However, USSR was a very illogical choice: at that point it was an ally of Germany and there was little reason to expect an easy victory.
• Another explanation is actively propagated by Victor Suvorov in many of his books. He describes facts speaking for his theory that Soviet Union intended to attack Germany and the German attack was indeed a preventive one. This isn't a new theory, the Germans actually used it already in their declaration of war on the Soviet Union. I'm not aware of any serious historians favoring this theory, and Suvorov unfortunately isn't one - he seems to be willingly omit or even falsificate facts in his books by misquoting and quoting out of context.

I recommend that anyone interested in this issue read Adam Tooze's "Wages of Destruction". This book is on the economic aspects of the 3rd Riech, from the economic history before the 3rd Reich, to how it conducted the war. But the book also engages in other aspects of German conduct of the war, including tactical and strategic reasons for this and that.

Tooze's book is the best explanation of the entire war, and it is fairly recently written history too.

Tooze' begins with what life was like for young Germans growing up in the last quarter of the 20th century, including a popular series of books that mesmerized German youth on tales of the American West, the conflict between cowboys and indians and the conquest of the West. Hitler would have had a hard time growing up and not being familiar with this. The period from 1870 to 1914 was a period of unprecedented economic growth and transformation of Germany, to the point where it eclipsed the United Kingdom in total economic output, (but not per capita output) especially in Steel, Electronics and Chemicals. This engendered pride for the German youth of Hitler's time. Germany constituted a sterling example of Western Civilization in many spheres of life, save politics. But when Hitler and people of like minds thought about the future, and looked at maps on the globe, it became clear that to continue to be a world power, Germany would have to take on continental proportions like China, Russia, perhaps Canada, India or Australia, and most definitely that of the United States, which as the Cowboy an Indian books illustrated, was taken forceably from the primitive people in the Native American Indians by an advance civilization, which increasingly was populated by ethnic Germans immigrating from Germany - at a rate of over 200,000 a year in the last third of the 19th century. To Hitler, for Germany to be a world power, it would need to have continental proportions, like that America got from the West. To this Hitler proposed Germany doing to the area east of its frontiers, what America did west of its frontiers, taking that land from what Hitler, and what many other uber-German nationalist viewed as an inferior peoples, the Slavic peoples of Poland and the Soviet Union.

What was driving this was long term, strategic economic interests and needs for economic resources. Russia had these.

The economic resource calculation drove events and decisions as even the discussion that Hitler had with Mannerhiem demonstrates, although I'm not sure if that discussion was not forthrightly honest, but taylored to Finnish ears.

Germany's strategic situation was little different than the UKs. It was an over crowded nation, that had a diet heavy in animal fats, that was too small to produce enough food of the kind it liked to eat. Prior to WWI, Germany had some Iron Ore to feed its industry in Alsace Lorriane. After WWI, those territories were lost and so all Iron Ore consumed by Geman Industry required foreign exchange reserves gained through manufactured export, only now, saddled with heavy reparations payments, such foreign exchange reserves were dear, hiegtening the strategic considerations Hitler's mind was already attuned to. So like England, Germany had an over crowded land, that had to export manufactured goods to pay for raw material imports. Unlike England, now Germany lacked a world class navy to ensure the steady flow of raw materials and food stuffs - meaning Germany was subject to interuptions to their economy, among others, by a British navy.

Much is made of Chamberlains concessions to Hitler at Munich. Tooze addresses some of this too. Prior to WWI, Europe was the center of the world, and the center of world power, and global economics. After WWI, the British and French were in massive debt to the United States while Germany's economy was highly dependent upon American loans. It was feared by both the British, Chamberlain specifically, and the French that another outbreak of a general war in Europe would have no other effect other than to make the European powers more fully pushed into 2nd rank powers dependent upon the flanking powers: the United States or Soviet Union/Russia. Chamberlain assumed that Hitler realized this too, and so at heart was not really interested in a new general war. Furthermore, Chamberlain fully believed that the Germans could once again be defeated by sound defence on land, and economic blockade that would bring Germany to its knees as what had happened in WWI. So to avoid a war that would render Britian a 2nd class power and satelite to the U.S., Chamberlain was prepared to sacrifice the Sudentenland as a way of throwing Hitler a bone and saving face. It was a crucial mistake. If Munich had not happened and Hitler had gone to a general war over the Sudentenland the German generals were ready to overthrow him. The outcome of Munich made that impossible. Shortly after Munich, Hitler cajoled the rump Czech state to request help in putting down an internal revolt, which gave Hitler legal pretext to send in troops, in effect conquering Czech without firing a shot. Stalin and the Russian saw the sell out of the Czech republic as firm evidence that the Western powers couldn't be trusted, so instead he chose to cut a deal with Hitler, the Nazi-Soviet pact. This pact was mostly economic, but it also divided up Poland which both powers saw as Repugnant. The Nazi-Soviet pact in a single stroke eliminated the threat any blockade would have on the Germany economy. Chamberlain's diplomacy was a total disaster.

Hitler's Germany, in Napoleon's parlance, occupied the central ground and proceeded to take on all surrounding powers in series, much the way Napoleon did in Italy and Germany during his day. He did this counter clock wise: First Poland, then Norway and Denmakr, then France and the Low countries, ideological friendly with Franco in Spain in the Southwest, alliance with Italy to his south, then invasion and attack of the Balkans to his Southeast. That brought him back to the issue of Russia.

Several things are added to Hitler's calculation. A sheer hatred of Communism and a belief it was decadent. The recent, massive purge of the Soviet officer corp. The demonstrated failure of effectiveness in the Soviet attack on Finland. Russia's poor performance during WWI, when it had what he would have thought of as a superior political system. Hitler's highest risk had been diplomatic move on the Sudentenland and Czech, Poland (because a more aggressive Western powers could easily roll into the Rhineland while the bulk of Germany's forces were in Poland), and then the attack in the West where the Germans faced superior numbers in men and material, even in tanks. What the Germans had going for them however was a superior battle doctrine, an excellent battle plan in the Manhiem plan, courtesy of the German general staff (and thanks to the original plan being exposed by a German plane accidentally landing in Belgium). All these other invasions, going into them, seemed like and probably were on the surface, higher risks. By the time Russia came along invading Russia seemed proportionately a much, much, much lower risks. Hitler had no idea that the Soviet people would react almost in unison to strive against him, this manifested in the miracle in the massive removal of Soviet industry to the Urals, nor could anyone know that the Russian's had a new tank, about to be produced in huge numbers that was superior to any that the Germans had. The reaction of the Russians was to fight and die in appauling conditions.

A side note is that, one possible reason Stalin refused to believe that the Germans would attack him in the run up to barbarossa, was that there had been no inflections in the price of wool in international commodities markets. You don't invade Russia without going out on world markets and purchase enormous quantities of wool for equiping your soldiers for Russian winters.

The reason Tooze gave for German declaration of war on the U.S. after Pearl Harbor, was because Hitler always believed that the U.S. would come in on the side of Britain sooner or later. Hitler had no antidote to the combined navies of Britain and the U.S., but Japan did have a substantial navy that could tie down one or even both. So Hitler continuously encouraged Japan to attack the U.S., and promised that Germany would declare war, as a sweetner, immediately after if Japan did. Japan did at Pearl Harbor, and Germany followed suit, as Hitler had promised.

One other side note is that, in the short run, instead of attacking Russia in 1941, a third of the resources used there, could have been applied to attacking Egypt, and a break through there, would have opened up the entire resources of the Middle East to Germany, most specifically, Oil. It would have required some accomodation with Turkey to best get the oil to Germany, but it would have rendered the entire Eastern Mediterranean to the Axis powers. This would have been nothing more than a continuation of the war with Britian. This is one of the big "ifs" of WWII.

German conquest of the Middle East would still have been a side show. Hitler's main strategic, long term plan was to take European and perhaps western Siberian Russia away from the Soviets, and Germanize it. And Russia was perceived as very weak at the time of the German invasion.

As for possibility of conquest: Russia was successfully conquered by the Mongols who controlled Russia for more than 200 years. The Poles had recently won a war over the Soviets in the early 1920s, extracting territory, which is why Stalin agreed to the division of Poland, to get back the land lost in 1920s. Finally, during WWI, Germany was signally successful on the Eastern front, but unsuccesful on the Western front. In WWII, it was just the reverse. Again, all of this made Russia look like a small risk. Too much reliance on hindsight? Perhaps.

Finally I dispute that Hitler insisted on the taking of Moscow. That was the goal of Guderian and the German Generals, but Hitler stalled the assault on Moscow and redirected Guderian south to form a pincer move behind the giant Soviet reserve armies in North Eastern Ukrain, in coordination with German armies of the South (Ukraine) moving north. This was successful, but it did delay the assault on Moscow -- which turned out to be just a smidgeon too far for them.

Production numbers Tooze assembles shows that Russia simply out produced Germany on most strategic weapons, and quite often in enormous numbers. That coupled to Soviet soldiers willingness to fight and die under impossible conditions, conditions that even the Germans shrank from, sealed Germany's fate in Russia.

Please note a few of the points here were not from Tooze's book, but by far most of them are. It's a good read, go out and get it. Economics explains much of the course of the war better than most other explanations.

The two big Nazi goals were the extermination of Jews and the conquest of land in the East, to be settled by Aryans, with the native populations drastically reduced in number and existing as uneducated servants. Hitler had to invade the Soviet Union. Also, I don't think anybody thought the peace between Germany and the Soviet Union was going to last: the Soviets considered that period as "creeping up to war".

The Soviet Union looked vulnerable. The Russian Empire had collapsed in 1917, despite not being the main target of the Central Powers in 1914 and 1916. Hitler thought the Communist regime would be weaker, and the German attack stronger (they'd conquered France in six weeks, after all), and that the Soviet Union would therefore collapse given a serious attack. Their performance in the Winter War against Finland didn't promise effective resistance, nor the purging of the Soviet officer corps. Both of those were potentially fleeting advantages. (Indeed, if the Germans had attacked in 1942, they would have faced 40 tank divisions, many of them well equipped and fairly well trained.)

Conquering the Soviets would also be very useful in the medium term. The Germans were suffering from the Allied blockade, and the Soviet Union had a great many useful resources.

Also, there was no good way of attacking Britain. The plans discussed seemed to revolve on conquering the Mediterranean first, and then bringing submarine and air power to bear in a long slow process. Once master of all of Europe, the Germans could start building an unbeatable navy, but that would take a long time. Better to have Soviet economic resources first. In the meantime, the British weren't going to hurt the Germans much.

Hitler knew that Churchill was looking for allies, and thought that the conquest of the Soviet Union would at least be a morale blow, leaving the British facing a long hard struggle with little hope of victory.

So, the plan was to attack hard by surprise, destroy Soviet border forces, and watch the Stalinist regime collapse. Once the hard fighting was over, and the German Army occupying vast stretches of land, troops could be demobilized to aid in the economy while war production switched to supporting the Navy and Air Force. (Some of this switchover actually happened, when the attack seemed wildly successful.) Britain could then be dealt with, if they weren't asking for peace terms yet.

Germany always wanted to attack and defeat Soviet Russia. There is an ideological battle between fascism and communism. Germany really thought that Russia was the enemy of the world. Some Germans believed, such was the evil of communism, that when they started the eastern front, the English would come over to their side to fight communism rather than continue fighting.

The western front was secure. Invasion of England was off the cards, but equally a counter invasion by England was thought unlikely, and at that time it was not possible for England to counter-attack in Europe. There was some expectation that the English would make a deal for the status quo in the west and not fight on further.

Russia was getting stronger. Every month delayed saw Russia prepare better, building more tanks, more airplanes and stockpiling arms. Any delay would have made the war in the east more difficult. Russia knew a major war was coming, either with Germany or Japan, and it had been working as fast as it could to rebuild its armies.

Germany expected the war in the East to be quick and decisive, just as the wars in central Europe and the West had been. They did not expect to be pulled deep into Russia and into the winter of Russia. They also made mistakes in strategy during the advance that meant that they couldn't achieve the goals that would ensure a quick victory, but they were sidetracked.

Russia contained significant resources of oil, coal, gas, steel, etc. Germany needed these to continue its war efforts. Without securing Russian resources, Germany could have been starved out.

By this time, Germany controlled the entire European peninsula, and it was very hard to see the Allied forces coming back from that.

Hitler told one of his generals in June 1940 that the victories in western Europe "finally freed his hands for his important real task: the showdown with Bolshevism" [from here].

Reasons to attack the Soviet Union include:

• Because having a massive military power right next door when you're fighting a war a thousand kilometers away made Germany nervous.
• To exterminate Communism
• Capture oil fields and other strategic resources.
• Lebensraum, or "living space", a core ideology of the Nazis.