What was the main cause of World War II
1. Franklin Roosevelt was elected president in 1940
2. The United States sold goods to its allies in Europe
3. The United States declared war on Germany and Italy
4. Germany began to expand its territory through conquest
4. Germany began to expand its territory through conquest
Expansionist policies adopted by Adolf Hitler and Nazi Germany, including the invasion of Poland in 1939, were a major cause of World War II. Hitler's aggressive actions and desire for territorial expansion ultimately led to the outbreak of the war.