The victorious allies imposed a punitive peace on German at the end of WWI. This caused huge resentment, that was at least partly responsible for the rise of Nazism and WWII. The Marshall plan was, I believe, an attempt to the stop the same cycle from happening again. Similarly for Japan. It was an act of incredible generosity by the USA, but I think history shows that it was also a very smart investment. I can't imagine such generosity and foresight being employed again any time soon.