The main factor that led to the start of World War II was the general unhappiness with the international status quo on the part of the Germans and the Japanese. Both of these countries felt that they were not being fairly treated by the international community.
For the Germans, the cause of this unhappiness was the Treaty of Versailles. The treaty assigned all the blame for WWI to the Germans. Furthermore, it punished them very harshly for the war. It forced them to pay reparations, to give up much of their territory, to reduce the size of their military, and to endure other humiliating conditions. Because of this, the Germans were eager for revenge and eager to change the international system. This is why they took the actions that started the war in Europe.
For the Japanese, the cause of this unhappiness was a desire for a larger empire. Japan felt it needed more of an empire to supply it with resources it did not have on its home islands. It also felt that it should have an empire commensurate with its status (in its eyes) as a great power. It was angry about the fact that it was not being allowed to have an empire by the European powers. For these reasons, it took the actions in China, Vietnam, and finally at Pearl Harbor, that started the war in the Pacific.
Thus, the overall factor behind the start of the war was dissatisfaction with the international status quo on the part of Germany and Japan.