World War II was caused by nationalism, militarism, imperialism, and unresolved territorial issues from World War I. Germany resented the Treaty of Versailles and persecuted Jews. Japan wanted to expand its military rule into Asia. The US initially pursued isolationism but increased support for Allies through arms sales and Lend-Lease Act. The US entered the war in 1941 after Japan attacked Pearl Harbor, bringing America into the fight against Germany and Japan.