(Copying from response my brother-in-law sent that was really helpful.)

As for the causal factors of WWII, there are many but a good place to start is the end of WWI: The Treaty of Versailles and war reparations;
Wiki Link
Another important causal factor of WWII to understand, one that I found very interesting as I learned more about it, was the role of geopolitics. The need for “Lebensraum” is often cited (Lebensraum can be roughly translated as ‘living room’ [for the German Volk]; you guys probably know this from your time in Germany but Volk is people. So, a Volkswagon is a “people wagon.”). This was actually an idea that developed over the course of about half a century going back to Sir Halford (sp?) Mackinder (MacKinder – UK) and Karl Haushofer (Haushofer — German). Rudolf Hess was a student of Haushofer, that is where many make the analytical connection between Geopolitics and the Nazi Party. Then, it’s not very difficult to make the analytical leap from the nationalism that is wrapped up in the idea of Lebensraum to how the Nazis were able to rationalize some of the things they did in WWII.