Why was the world plunged into world war 2 in 1939

The world was into war in 19239 because Hitler started to want to take over Poland but England and France warned him that if would they would declare war. But he invaded anyway and they declared war. Hitler had also made a pact with the U. S. S. R but when he broke it they declared war. Then China joined because Japan who was allies with Hitler invaded it. Also Japan bombed Pearl Harbor and the U. S joined. Then parts of Africa and Australasia joined because they were colonies of the allies.