The four main causes of World War II are the Treaty of Versailles, Hitler's actions, appeasement, and the League of Nations.
The Treaty of Versailles was a peace treaty that ended WWI. Germany had to pay reparations from the war, and had to limit their military. They also had to give up land and take full blame for the war which caused them to become angry.
The league of nations was a peace keeping body comprised of several countries after WWI. In the end the group was useless due to lack of military power. The US did not participate in the group because they became isolationists.
Hitler continued to build his military, and invade territory. Fresh out of WWI no countries wanted to start another war, so they appeased hitler and gave into his demands.
Hitler was a dictator who controlled everything in Germany. He blamed other countries for blaming Germany for WWI. Hitler also believed the ideal race was the Aryan race, and blamed the Jews.
All these causes eventually led to the second World War, in which Hitler continues to mass murder, and the world once again reaches mass chaos.