FDR was of the opinion that the Germans needed to take a terrible beating in World War II if they were to be peaceful in the future. He was right. I think that our policies after World War II clearly worked better than after World War I, but after World War II, Germany couldn't be under any illusions that they hadn't been beaten. The war was brought home to every German in a way World War I never was. So I have my doubts that a more magnanimous policy after World War I would have worked as well as it did after World War II. I suspect that they would have simply taken advantage of it, and started another war anyway. They needed the beating they got to turn the lion into the lamb.
You can apply pretty much every word of this to Japan, too.
It took their utter and complete defeat at the hands of the United States, the worst defeat of their entire national existence, to stop what they had become.
