What was the main cause of WW II?

war in Europe was caused by the German invasion of Poland and the war in Asia was triggered by the Japanese invasion of China.

The inmediate causes of WW2 was:

Hitler invaded Poland bringing France and Great Britain into the war,Japan took over Manchuria bringing China into the war,Japan attacked the US Fleet at Pearl Harbor bringing the US into the war, and Germany invaded the USSR bringing the Soviet Union into the war.