The only remarkable thing is that we treated Japan so well, considering how its fellow far east countries were treated as client states or worse for so long.
I think it was latent guilt of using the nuke on them.
So they had an industrial base. No so much for (S) Korea and VietNam. They were very agrarian (like Japan before they built up their war machine). So their problem was peasant farmers with no land + a growing population on the brink of starvation.
US advisors made recommendations to alleviate this problem by doing land redistribution for tenant farmers. South VietNam would have none of that. Korea did appease the peasants albeit reluctantly so they avoided the worst.
The US has a pretty good track record when it comes to building insurgencies though, remember that the Taliban were built and trained by ISI and the CIA. Iran had islamic groups that the Shah used to silence opposition forces. There's no way their training and equipment wasn't done in coordination with their US partners given how much of a puppet the Shah was. Or take a look at a lot of the groups that were part of the opposition in Syria.
I wonder if there's any analysis why the US has been so good at building and training insurgency groups, but so bad at making local armies to protect their own territorial borders.
You though, but you did not did.
In both prior cases US ran moderately competent occupation government, and by some accounts one better than their own.
The model US followed in Vietnam was not this. Ngo Dinh Diem was a Syngman Rhee 2.0, but even worse. Here, you can say Karzai was Syngman Rhee 3.0.
All shared a history of being overthrown because of progressing nationalist drift within their own power bases.