The two main reasons for the beginning of the Second World War were: the Munich agreement (which was signed in 1938, between Germany, UK, France and Italy) and the invasion in Czechoslovakia (March 15 1939) by Hitler. Although, I think the most important cause for the beginning of world war two was the invasion in Czechoslovakia.
A few years after marching in Rhineland, Hitler's next target was to get Sudetenland into German territory. Since Sudetenland was in Czechoslovakia, the first thing he did was to gather some Nazis there and get them to revolt against the Czech government.
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