Answer:
France.
Step-by-step explanation:
France fell to the Nazis in 1940, only one year after the start of the war. The Battle of France was relatively easy for the German army, because France was not prepared, while Germany easily swept through Belgium, Northern France, until capturing Paris.
After the decisive German victory in France, Hitler felt confident about his plan to invade the Soviet Union. His intention was to either enslave, or slaughter most of the slavic population, in order to maker room for a new German Colonization.
The idea ended up being disastrous because the Soviet Union would ultimately be the decisive winner of the confrontation in 1945.