WWI was a thing.
Can you expand your comment? I am not sure what you mean by such a short reaction. Of course there was WWI, but ... ?
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.
I think the Cold War played a role too. Germany was defeated, but its industry was still fairly operational. In contrast to housing, which was bombed into pieces, German heavy industry was remarkably operational until the very end and the British were surprised to find that German factories were, on average, equipped with more modern machines than British ones.
And some 75 per cent of that industry was in Allied hands. It made strategic sense to rebuild the country in face of a Soviet threat and make it a factory for the Allies (notably, the German army was only reconstituted much later, in 1955) instead of destroying it.
>I think the Cold War played a role too.
Yes, definitely. The USA wanted as many allies as it could get against the USSR, even former enemies.
>German factories were, on average, equipped with more modern machines than British ones.
Thankfully the Germans wasted amounts of resources on not very useful weapons, such as the V2 rocket (technologically brilliant, strategically pretty useless) and the King Tiger (unreliable and IIRC cost somewhere around 20 times as much to make as a typical allied tank).
Due to a somewhat historically anomalous generosity of the winners, who (from a mixture of humanity and economic motives) decided to invest into its rebuilding.
In earlier times, debellatio of an enemy state after a long, vicious existential war would end in a way similar to what the Romans did to Carthage.