What caused world war two to breakout?

Hitler's invasion of Poland in September 1939 drove Great Britain and France to declare war on Germany, and World War II had begun. Over the next six years, the conflict would take more lives and destroy more land and property around the globe than any previous war.

Aggression between countries were very high. Hitler and nazis were starting to grow and Hitler became leader of Germany. Italy and Germany started being aggressive with other countries who did not want to fight, because ww1 just finished