I think the biggest cause of the war was Hitler's invasion of Czechoslovakia.
There are many reasons that caused the Second World War. The main figure here was Adolf Hitler. He rearmed Germany, remilitarized Rhineland, took over Sudetenland, invaded in Czechoslovakia and finally pushed Germany, Europe and even the whole world into war.
Among the several things he did, taking over Sudetenland and Czechoslovakia are two main causes of the war. In March 11, 1938, Hitler invaded Austria. The aim of this invasion is to get Sudetenland "back" to Germany. It also followed what Hitler said in Mein DKampf: "to include all German-speakers in the Third Reich, especially those in Austria and Czechoslovakia." People in Sudetenland were willing to join Germany because most of them were once German and speaks German. In this way, Germany easily took over Sudetenland, and Britain and France did nothing. So in the same way, Hitler took over Austria as most of the Austrian wanted to join Germany.
However, this action had broke the Treaty of Versailles because an anschluss (union with Austria), but Britain and France did nothing. Further more, on September 29, 1938, they agreed to give Hitler Sudetenland under the Munich Agreement. Now Hitler's owned Sudetenland and Austria and no one had even tried to stop him. So he demanded more and more and finally caused the outbreak of Second World War. However, if Britain and France had done anything to stop Germany, the Second World War may never have happened because at the time, Britain and France had the ability to defeat Hitler. They could easily got Hitler out of Austria and Sudetenland as they've got far more solders and bigger navy. But they did nothing. They simply thought that Hitler would stop expending when he got what...