Not, apparently, to make the world safe for democracy:
As a result, we liberated one half of Europe at the cost of enslaving the other half for fifty years. We really did win the war against one genocidal dictator with the help of another. There was a happy end for us, but not for everybody. This does not make us bad—there were limitations, reasons, legitimate explanations for what happened. But it does make us less exceptional. And it does make World War II less exceptional, more morally ambiguous, and thus more similar to the wars that followed.
And also the wars that preceded it.