Most of the world, I think, went to war against the axis powers because they were attacked by axis powers, so the question is "what did Germany and Japan think they could get out of starting this war?" and the best answer I can come up with is Norman Ohler's book on it;

I mean, I can see why the allied countries decided they needed to throw all they had into the fight... I just don't see how Germany could have actually done the math sober and thought they could win a war against... almost the rest of the world.