The Treaty of Versailles blamed Germany for WWI and forced them to pay extremely high reparations to the United States, Britain, and France. Germany was in turmoil, and the oppression caused by the treaty created a lot of hate for the three countries.
Fascism began spreading in countries like Italy and Germany because after WWI governments were in dismay. Tyrants like Hitler and Mussolini came to power with false promises of glory for their broken countries.
League of Nations
Directly Disobeying the treaty of versailles Nazi Germany, led by Hitler, began constructing an army and developing new war tactics like blitzkrieg. Germany began laying claim to all German speaking lands and were met by no opposition. Key Contributor to the war was Germany's invasion of Poland.
Munich Conference held between Germany and Great Britain. Germany demanded Sudentenland. In order to prevent war Britain appeased.
The league of nations consisted of all major nations except for U.S. it wanted to stay isolated. The league of nations was ineffective because it had no military to enforce its rules and regulations and did nothing to stop Germany.