These are some theories to how WWII started. There have been many theories flying by, such as the invasion of Poland, the invasion of Ethiopia (by Facist Italy), and the invasion of both Manchuria and China. The goal behind the invasion of Poland was because Hitler wanted to conquer the countries around Germany, and make them German. Then he would use those forces and prepare for a full scale war, but there was no specific reason why Poland was first.
The reason for the invasions of China and Manchuria was because Tojo (or Tojo-"sama" by Japanese honors system), was so they could bring civilians to those two countries, so it wouldn't be as overpopulated. The invasion was called "seisen" or "holy war", but then followed by Ninking or Ninjing massacre. Ninking massacre was when soldiers of the Empire (sounds so Star Wars like) raped, looted or executed the prisoners. For this reason(and a lot more) was why the Geneva Convention was founded.