Final answer:
The leading cause of World War II was Adolf Hitler's personal obsession with creating a vastly expanded German empire. The war was also influenced by the worldwide reaction to the Great Depression and the rise of fascism and authoritarianism in Europe and Japan. Japan's attack on Pearl Harbor led to the United States' entry into the war.
Step-by-step explanation:
The primary cause of World War II was Adolf Hitler's personal obsession with creating a vastly expanded German empire. Hitler's aggression and desire for power led to Germany, Italy, Japan, and their allies, known as 'The Axis', launching a war of aggression. Other contributing factors included the worldwide reaction to the Great Depression, the rise of fascism and authoritarianism in Europe and Japan, and Germany's manipulation of diplomatic situations. The United States entered the war in 1941 after Japan's attack on Pearl Harbor.