What caused the 2nd world war

1 Expansionism

2 Militarism

3 Germans vs Slavs

4 Interrelations and economics

4 Problems with the Treaty of Versailles

4.2 French security demands

4.3 Japan's seizure of resources and markets

4.4 Failure of the League of Nations

4.5 The Mason-Overy Debate: "The Flight into War" theory

5 Specific developments

5.1 Nazi dictatorship

5.2 Re-militarization of the Rhineland

5.3 Italian invasion of Ethiopia (Abyssinia)

5.4 Spanish Civil War

5.5 Second Sino-Japanese War

5.6 Anschluss

5.7 Munich Agreement

5.8 Italian invasion of Albania

5 Soviet-Japanese Border War

5. Danzig crisis

5. Invasion of the Soviet Union

5. Attacks on Pearl Harbor, the Philippines, British Malaya, Singapore and Hong Kong

Hitler's invasion of Poland is what set off the spark that ignited the war (European countries declared war on Germany, Germany and it's allies declared war on other European countries, and so on).