There were many causes of World War II. The most significant cause was the rise of Nazism in Germany. This itself had causes reaching back to the end of World War I and the Treaty of Versailles, which, in combination with the Great Depression, left the German people embittered and paved the way for ultra-nationalist leader like Hitler. Another cause was the aggressive actions of the militarist regime in Japan, whose invasion of China many consider to be the real beginning of World War II. The weakness of the League of Nations, which had been established in the wake of World War I, was another factor, as the League proved incapable of restraining the Japanese or Mussolini's aggression in Ethiopia.
Another factor was the policy of appeasement pursued by Britain and France, which emboldened Hitler. American isolationism was a factor encouraging appeasement, but the United States's strategic presence in the Pacific was a factor that contributed to making the war truly global, as American interests inevitably clashed with Japanese aggression. Another factor was, of course, the event which actually began the war, i.e., the German invasion of Poland. But this event may not have happened if the Soviet Union had not agreed to a non-aggression pact with the Nazi regime. So there are many complex and interlocking causes of the conflict.