It's a lot easier to win a war against a state than a war against a people. With wars like in Vietnam, the US was not merely fighting a state, but Eisenhower himself said over 80% of Vietnamese supported the communists. Guerrilla armies would constantly pop up and people would spontaneously fight back just because the South Vietnamese government was so overwhelmingly unpopular.

The more Vietnamese the US killed, the more joined the communists, until the US resorted to using chemical weapons to try and destroy the food supply and starve them to death, and still couldn't win, or in Korea where the US destroyed every standing building to the point that they began dumping bombs in the ocean because the bombers couldn't find enough targets anymore and needed to lose the weight to land.

The US could not have won the Vietnam war without actually nearly completing their genocide. Same with Korea, and even Afghanistan. The more they fought, the more people joined the other side to fight against the US because they just wanted an independent country and didn't want a foreign colonizer deciding for them.

When the US overthrew the fascist Japanese state, and aided in overthrowing the fascist Nazi state, they replaced these states with new ones that they retained a lot of the original institutions and a lot of the original people in power. There was a change, but the change was not that fundamental, the US did not have to kill off most the Japanese population or the German population to make these changes, and their mass killings of civilians they did participate in was not necessary.

The situation with Vietnam, Korea, Afghanistan, Cambodia, this was entirely different. The US was not just trying to take control and make minor changes to the state. The US was trying to change the entire people, they wanted to purge communist and national liberation ideas from Vietnam, when these ideas gripped the masses. They had to fundamentally change the people themselves, which put the US at war with the people, which was a lot more difficult of a war to win than the war with Japan or Germany, even though Vietnam is a smaller, poorer country.