What is the reason why world war 2 happened?

Answer 1

I consider the rise of the Hitler-Regime and his NSDAP as the most important reason. Germany was down after World War 1, it's first trials of installing a democratic system failed (Weimar Republic), the worldwide economic crisis in 1924 and 1929 provoked poorness and huge unemployment. Especially the restrictive punishments of the "peace"-contract of Versailles, the fall of the Prussian Military Regime and the growing popularity of socialistic ideologies (or the thread of it) made a lot of people vote for Hitler, who promised to give back to the people it's pride.

Hitler was elected in 1933 and new well to expand his power and popularity (e.g. by claiming minorities guilty; see "fascism. The ww2 started with the "Blitzkrieg", the invasion on Poland on 09/01/1939. Before, the Allies forces of the contract of Versailles didn't invented as Hitler reestablished the Wehrmacht, garrisoned the Rhineland (1936), then Austria (1938), parts of Czech, Slovakia (Sudetenland, 1938), Romania (Banat, Transylvania, 1938) and Russia (Memelland, 1939).

Some say that the ww2 started even before 1939, because the crisis between China and Japan (Manchuria) started in 1931, other say the ww2 started later, when the war really became global with the attack on Pearl Harbour in 1941.