Was there a nation/state which conquered other ones to gather forces and fight against an even greater enemy?

What about Japan in the WW2 period? Gathering up islands to the south such as Singapore. Then they acquired POWs who became slaves, while getting resources such as rubber. All to compete with a bigger enemy, the US.

Nazi Germany conquering all of Europe and using French, Norwegian, Danish, dutch, Belgian, Czech, Austrian, etc and even Yugoslav troops

Too many to count. In addition to those in other answers:

• The Republic of Rome - conquered Italy and under Marius enfranchised Italian legionnaires following twenty years of service.

• Philip of Macedon united the city-states of Greece; his son Alexander conquered the Persian Empire with a unified army of Macedonians and Greeks.

• Alfred King of Wessex united the Saxon petty kingdoms of England into a nascent version of that state to repel the Danish invaders.

• Bismark, Chancellor of Germany, used the threat of Austrian and French hegemony to unify the petty states of Germany into the Second Reich under Kaiser Wilhelm I.

• Tecumseh and his brother, the Prophet, attempted to unify the tribes of the Ohio Valley and Lower Great Lakes basin as allies of the British against the United States.

• Sitting Bull attempted to unify the Plains tribes against the United States - though he also failed.

• The Commancheria may be another example from 19th century U.S. history.

• Barbarossa's (re-)unification of the Holy Roman Empire after a long civil war prior to joining the Third Crusade against Saladin.

• Charles Martel fought a long civil war to unify the Franks - just in time to defeat the Moors at the Battle of Tours.

• The Iroquois Confederacy conquered everything South of the Great Lakes from Illinois to Upper new York State and Maryland, as well as SW Ontario, as part of their war against the French.

• Hitler's occupation of the Rhineland, anschluss with Austria, and absorption of the Sudetenland prior to World War Two. These regions, unlike those conquered afterwards in Western, Central and Eastern Europe, were eager participants in the Wehrmacht and S.S. during World War Two.

Some may argue that a few of the above were accomplished without blood being shed in battle - yet the great military theorists have always recognized that the greatest victory is exactly that - one won so decisively in advance that the enemy succumbs without battle:

The greatest victory is that which requires no battle.
Sun Tzu - The Art of War

War is the continuation of politics by other means.
Carl von Clausewitz - On War

To give a specific example from Roman history, the Social War (91-88 BC), uniting Italy as a political entity (as opposed to a patchwork of allies and colonies), was immediately followed by war against Mithridates VI in Asia.

For me the classic example is Edward I of England (aka "Edward Longshanks").

He conquered Wales and once pacified, combined the English and Welsh forces against the Scots thereby earning the epithet "Hammer of the Scots".

There are many cases, feel free to add more:

• Napoleon conquerer north of Italy, and recruited forces there for other wars. Later on, after defeating Austria and Prussia, Napoleon invaded Russia. The invasion of Russia was not only with french forces, both flanks of his advance where covered by prussian and austrian forces (north and south).
• As @Steven Burnap says. Genghis Khan is another good example. He unified mongol tribes by war, and with them he invaded other countries. Later on, even chinese engineers where included in his armies fighting in Europe.
• In WWII, Soviet Union used romanian, finish, polish and bulgarian armies as allies at the end of the war against Germany, even though they were enemies (or almost enemies) before.
• Spanish conquest of the new world. Spanish forces were quite small, they actually allied with other tribes to defeat major empires, these states or tribes joined spanish conquistadors either by convenience or defeat. Aztec empire did not fall to a small spanish adventurers.