What were the primary causes of World War II in Europe? The war began in Asia in 1937, in Europe in 1939, and the USSR and USA both joined in 1941 after suffering attacks. Why was WWII the worst war in human history? Consider the 27 million Soviet people who died, 6 million Jews, and millions of other people, from Poles, to Czechs, to Roma, etc. The Red Army played the primary role in defeating Nazi Germany (inflicting 80% of the casualties against the Wehrmacht) while the US served as the main provider of goods to its Allies (UK, USSR), and led the victory against Japan. Why did these realities lead to the Allied victory in the war (Germany did not lose, the Allies won)? Why did Nazi Germany, its allies, and collaborators perpetrate the Holocaust? There is no one reason but a whole host of interconnected reasons. What role did the Holocaust play in the creation of Israel?