World War II is a continuation of World War I and the cause of World War II can be pointed directly at Hitler... His ambitions to restore Germany to its glory days and rebuild a Thousand Year Reich is what got the whole thing started. That and also that the other Western powers were reluctant to get into another war with Germany (because of their previous experiences in World War I).

The League of Nations would have benefited if the USA joined it, but at time America was practising an isolationist policy of not getting involved in European affairs.