A variety of reasons. Some of them were more noble (e.g. not wanting to leave Japan in a terrible state), but most of them were at best enlightened self-interest.
Building Japan back up, but better, is likely to create a long-term ally, rather than ensuring a long-term enemy if the US left them as they were. The US had seen what horrible damage the Treaty of Versailles had inflicted on the German Empire, and its leadership wanted to *avoid* setting up the conditions to empower yet another fascist dictatorship. Investing into a large, industrially-capable nation could make the US a hell of a lot of money if Japan actually recovered (which they did, and yes, it was a massively successful investment.) The US at that point was, and to this day remains, the only nation to ever use a nuclear weapon in war–rebuilding Japan was a way to improve our international reputation. Having a Japan that is relatively prosperous, independent, liberal-democratic, and capitalist would ensure a base of operations for US military activities all across eastern Asia, which was a big deal given the ongoing tensions with Russia and China.
Ultimately, they did it because it make economic, political, diplomatic, and military sense. That it was also a generally humanitarian thing to do was a nice bonus–but it probably would have happened even if that weren’t true.
Latest Answers