What key factors led to the start of WWII?

The main
factor that led to the start ofwas 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.