I believe that World War II began because Germany pushed their limits. Hitler used the abrupt limits of the Versailles Treaty to take over land where there were German speakers. By September of 1938, Germany had already successfully acquired Czechoslovakia and Austria. These two countries weren't enough for them. I think they were absolutely crazy for trying to push their luck any further. They had achieved something that was already so difficult. Doubling your money is always the risky choice.
As for France and Great Britain, not wanting to repeat the huge losses they experienced in World War I, they decided not to attempt stopping Hitler from taking over Czechoslovakia and Austria. After Germany realized what they could get done, September 1, 1939, they decided to go ahead and take over Poland as well. After France and Great Britain saw that Hitler/Germany wasn't going to stop after that, they gave Germany the option of war.
After Germany declined the alternate route, France and Great Britain knew what they had to do. Two days later, they declared war on Germany. If Hitler and Germany hadn't been so power hungry, France and Great Britain would have never had to even consider the option of war. Germany had already conquered two countries; going for the third was truly the definition of going too far. It’s almost like getting tagged out between third and home in a game of baseball. Looking back on everything that took place, without a doubt, Germany just going too far is the main cause of World War