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.
With the help of the Nazi followers in Austria, in 1938, Hitler invades it. After the invasion in Austria, Hitler manages to get France, UK and Italy to sign an agreement with him giving him Sudetenland. I think this is one of the most important cause for the beginning of the Second World War, but not the main one, because there are certain events that follow this one which precede world war two
After having Sudetenland, and after invading Austria, Hitler decided that he wanted the whole of Czechoslovakia.
First he threatened war on it.
So, on March 15 1939, Hitler marched into Czechoslovakia and managed to occupy it. After this event, UK and France realized that Hitler's target was to conquer all Europe by force. This is probably the main event that caused the start of world war two, because after realizing Hitler wanted the whole Europe to be German, UK and France knew that the only way to stop him was to begin a new war.
The conclusion for conquering Sudetenland and afterwards invading Czechoslovakia was world war two. Even though I think that the main event for the beginning of world war two was Hitler occupying the whole of...