The war was caused by both long and short term factors.
From October 1929 an economic depression, starting in America, spread across the world. Millions were thrown out of work – some starved to death.
People were angry, and they turned to political extremists:
Countries became more aggressive:
Hitler built up a Greater Germany in central Europe. In 1938 he united Austria and Germany, known as the Anschluss, and demanded the Sudetenland, the German-speaking areas of Czechoslovakia. In doing so, he broke the terms of the Treaty of Versailles.
Faced with aggression like this, the League of Nations was powerless.
World leaders like the British Prime Minister, Neville Chamberlain, and France's Édouard Daladier tried a policy of appeasement – negotiating with Hitler and offering concessions.
In September 1938, the Munich Agreement gave Hitler the Sudetenland. The Czechs were not asked what they thought about it. Hitler occupied the Sudentenland in the following month.
Appeasement failed to stop Hitler: