Some of the causes of World War II were Italian fascism in the 1920s, Japanese militarism and invasions of China in the 1930s, and especially the political takeover in 1933 of Germany by Hitler and his Nazi Party and its aggressive foreign policy.
why world war 2 happened is because of the anger felt in Weimar Germany that was caused by the treaty of versailles and also another cause was the obvious inability of the league of the nations to deal with major international issues