The causes of the world war 2 include
Germans unhappy for the treaty of versailles 
the invasion of Germany in Poland
rise of the nazis
japanese invasion of china
failure of the league of nations

The immediate cause for world war 2 was the German tanks driving into poland on sep 1 1939. Hitler invaded Poland as punishment for refusing to handover the port of Danzig to Germany.Polish territory lay between two parts of Germany . Hitler thought the takeover of Poland will also be a small military operation.But Poland had a defence pact with britain which jumped on to polish side and France joined them a day later . This marked the beginning of worl war 2