In WWII, what was behind the very early decision to accept nothing but unconditional surrender in 1941-42?

One account that I read was that Churchill and Roosevelt agreed that although Germany lost WW1, the homeland had not been invaded and the German people had not suffered during the war and this is the reason Germany was so easily led into initiating another war just 20 years later. So Churchill and Roosevelt decided that the German homeland must be invaded and destroyed so that the German people would suffer the ravages of war and lose all appetite for another in the future. Thus, they agreed on requiring unconditional surrender to ensure this goal.

I always thought of Unconditional Surrender being applied to the Japanese. It never really occurred to me that it applied to the German side of the Axis, too.

One of the causes of the friction leading up to the war was that Washington and London were trying to arrange the world so it favored their nations and national interests. The Japanese (and many other nations) were not given a seat at the Big Boy Table. The Washington Naval Treaty set Japan's maximum tonnage of warship to 60% of each of the US's and the UK's. This was to guarantee that Japan could not push the US or UK out of the Pacific. Further, since the US and UK are pals, the Japanese could count on being out-gunned by a factor of 3. (The 60% value was arrived at with the help of US signal intelligence and code-breaking. We knew the lowest number the Japanese government would tolerate.)

Now we have royally cheesed-off an extremist faction of Japan's government, and this faction grows in power over time.

So, Pearl Harbor.

Japan is fighting for integrity and a Pacific empire. Circa 1940 USA doesn't give a hoot about what they think their integrity is, and Washington doesn't want them to have an empire (wisely).

Finally with that setup, on conditional surrender:

On Dec 7th, 1941 the Japanese killed 2,400 citizens, destroyed 8 battleships, several small vessels, and damaged a naval base. Let's call it a treacherous sneak attack. On the same day similar mischief happened all over the Pacific Rim, with great loss of life.

What do you think of the idea that a people we consider a 2nd rate power does this, loses WW2 at Midway (Game over, man, game over!), and then is granted surrender terms that allows them to solidify and legitimize their budding empire? The same empire we spent 20 years trying to prevent? It's not happening. If it did, it would have been a strategic victory for the Japanese.

Another reason relates to WWI. The UK, France and Russia had all been badly damaged by that war, and here they were fighting another one in which Germany was the most dangerous opponent.

The idea of doing it all over again in another few decades was deeply unattractive. Winning the war totally and remaking Germany so that it would no longer be aggressive offered the possibility of averting that.

Another purpose was to make a separate peace more difficult. The British and the Russians would have been in significant danger if the other made a separate or early peace with the Germans. The US Russia or England could in theory have signed a peace with Germany and left the war and left the remaining allies in a great deal of danger. This type of peace would have to have been conditional ie the German government would have be left still standing.

While I'm not claiming that there was a huge risk calling for unconditional surrender was a way for the allies to reassure each other that they were all in it for the long term.

To add to @TomAu's excellent answer, the opposite of unconditional surrender, "condidtional surrender" would have essentialy implied that some faction of the Nazi government would be allowed to carry on, after the removal of Hitler and his principal cronies.

Purely speculatively, of course, Speer or maybe Baldur von Schirach come to mind as possible post-Hitler Nazi leaders.

Of course, this was totally unacceptable to the Allies. In such a scenario the Nazi crimes would have not been repudiated and atoned for, but rather downplayed. It would have meant that all the fighting had really been in vain.

Another argument I have heard in relation to it was in relation to the stab in the back myth.

In 1918, Germany was on the brink of collapse: the economic blockade caused great sufferings, its allies were beginning to falter, and the USA was pouring more and more men on the field. The last Spring offensives, designed to deliver a decisive victory, had failed to do so.

So they accepted an armistice, retreated from France and interned the fleet at Scapa Flow, under British control. There was a negotiated peace, but the allied conditions were pretty harsh.

Now, some people (principally in the military) did not take it well¹. After all, certainly the German Army had lost territory towards the allies and had suffered severe defeats, but there was still a German Army, there had been no "decisive battle" to take it out and Germany itself was still free of enemy forces. To them, signing the armistice was an error because they though that the war could still be won or, at least, a better treaty negotiated while the Army was still in France and Belgium.

This developed into the "stab in the back myth", that the Army had been betrayed by the civilian government². It also allowed for the more militaristic faction to claim that Germany could have won the war and that Germany should try to recover the lost territories by military action.

Now, some claim³ that an objective of unconditional surrender was avoid a repeat of that: Germany defeat had to be absolute, to eliminate any possibility of anybody defending the German policies that led to the start of the war.

²The Nazis would later use it to single out Jewish and leftists as responsible for their defeat.

³See the Wikipedia page.

Churchill went over this a bit in his autobiography of the war. The idea was Roosevelt's, and it harkens back to the US Civil War and the Battle of Ft. Donaldson. General U.S. Grant used the phrase when the Confederate commander asked for his "terms".

Sir: Yours of this date proposing Armistice, and appointment of Commissioners, to settle terms of Capitulation is just received. No terms except unconditional and immediate surrender can be accepted.

I propose to move immediately upon your works.

Union propaganda sized upon this, turning it into something close to a modern meme (picture it getting tens of thousands of retweets with this video attached). For example, it was said that "Unconditional Surrender" was actually what Grant's first two initials stood for.

The meme had so much staying power, that Roosevelt pushed for it as the official position of the allies at the Casablanca Conference. Stalin likely didn't care as much about the US press reaction as Roosevelt, but he wasn't exactly a soft man himself, so he didn't have any objections to it. Churchill would likely have found himself outvoted, if he'd cared to press the issue. He didn't.