The war had a definite cause, unbridled military expansion by Germany, Japan, and to a small extent Italy. Now, it's easy to claim that Hitler was crazy or evil, and in fact he was certainly both, but that doesn't explain the Nazis' decision to invade Russia, and it sure doesn't explain Japan's decision to bomb Pearl Harbor. And there are many possible explanations beyond mere evil, but the most interesting one, to me, involves food. Hitler had a number of reasons for wanting to expand Germany's territory, but he often talked about Lebensraum, or living space, for the German people. German agriculture was really inefficiently organized into lots of small farms, and that meant that Germany needed a lot of land in order to be self-sufficient in food production. The plan was to take Poland, the Ukraine, and eastern Russia, and then resettle that land with lots of Germans so that it could feed German people. This was called the Hunger Plan, because the plan called for 20 million people to starve to death.