I dated a Japanese girl in college, and the subject of WWII somehow came up. I was curious about what Japanese history classes teach about WWII, so I asked her why Japan attacked Pearl Harbor.
She said, "There was no reason. We just attacked." That seems overly simplified to me, but I haven't heard much about the oil and steel industry in Hawaii.
When one man has absolute authority over a country, and he is considered to be their god, I can see how he would start to think that he deserves more. It doesn't really matter whether he has a rational justification or just an overly inflated view of his importance, IMO.