You can look at World War II as a battle between democracy and fascism. You would think that fascism would be more conducive to the war effort, since you can force people to work for you. Yet America, a democracy, managed to be productive through moral imperative and patriotism.
My first impulse is a moral one. I'm tempted to respond with an answer along the lines of who was on the "good side" and who was on the "bad side". The way my schooling cast this war is very much the same as it continues to be cast in American films - there were the good guys, i.e., the Allies, and there were the bad guys, i.e., the Axis powers.
However, we can look at the war in other ways, one of which is suggested in the post above. WWII can be accurately cast as a war of industry. Alternatively, the war has been cast as a determination of a bureaucratic mode of state and global governance: capitalism, socialism or communism.
Seen in this last way, Churchill's suggestion that WWI and WWII should be seen as a single conflict seems to apply. Capitalism won, when seen in this way, and perhaps this turned out to be the case because of capitalism's specific relationship with industry.
I think that the main thing that was going to determine who won was industrial power. This was an industrial war and the United States was by far the biggest industrial power, particularly since no enemy could attack its factories.
That said, Germany could easily have won the war if it had been content with what it had in early 1941. The US would not have gone to war to retake France and England alone could not have defeated Germany.