The major causes for the start and development of the World War II are the expansionism, nationalism, and natural resources.
This war was started because of the territorial appetites of some countries, namely Germany, Italy, and Japan, each them having their own vision and primary goal. Germany wanted to take over all the historic territories where the Germanic people lived, Italy wanted to resurrect the Roman Empire, while Japan wanted to get hold onto parts of Asia that were rich in natural resources as they have very few.
The nationalism led to mass murders on ethnic basis, with Germany and Japan being the biggest contributors to that, killing all the people that they considered as lesser human being or not suitable.
The natural resources were also one of the main reasons. Germany was trying to get to multiple places that were rich in natural resources, especially with oil, which is why the Soviets were attacked, so that Germany can get its hands on Azerbaijan. Japan too wanted and managed to conquer territories that were rich in oil and metals, as well as other natural resources, providing them with basis for further expansion and strengthening.
