> At the time, the non-communist regimes they were propping up in Korea or Vietnam were hardly any less brutal than their communist opponents as far as human rights are concerned (of course I'm not talking about the Khmer Rouge).

Tell that to the hundreds of thousands of refugees who fled South Vietnam, or the massive crowds in Saigon begging the Americans to bring them or at least their children along as they evacuated the US embassy by helicopter.

> Geopolitically it would have made perfect sense for the US to back the reunification of Vietnam under a socialist/moderately communist regime. The USSR had limited ability to project power in the region (unlike in Korea) and Vietnam would've certainly sided with the US instead instead of China despite the ideological differences.

Vietnam sided with the USSR over China; their split with the Khmer Rouge was largely a proxy for the broader Sino-Soviet split. Nixon and Kissinger, along with their mistaken policy of pulling out of Vietnam, also adopted a mistaken policy of trying to cozy up with Communist China to try and widen and exploit this split. Our relatively good relations with Vietnam today are largely a consequence of the Soviets not being around anymore and China emerging as a major threat, both of which were largely unforeseen in the 1970’s.

The Carter administration actually adopted a policy very similar to what you recommend, of withholding support for authoritarian regimes otherwise friendly to the US who faced too much popular resistance. It didn’t work out well for reasons Jeane Kirkpatrick explained very clearly at the time: https://web.archive.org/web/20110228072902/http://www.commen...

> What do you think US could've done different in Vietnam (besides directly invading the north)? Not a rhetorical question, I'm genuinely curious because to me that was seemed pretty hopeless.

It might have seemed hopeless, but we were losing the propaganda war much more than we were losing the actual war on the ground. The Tet Offensive was a huge military disaster for the communists but got spun and misrepresented as a disaster for the US. This isn’t to say that there weren’t blunders on the part of the US—McNamara was one of the worst SecDefs we’ve ever had—but despite that, we were still winning.

Back in the Civil War, the South’s basic theory was that, while they could never beat the North in a straight up fight, maybe they could make the war so long and bloody and miserable that eventually the North would get tired of the war and lose its resolve. And to the South’s credit, the war was long and bloody and miserable. The Union made plenty of blunders. Some of the bloodiest days of American history were the days when rebel incursions into the North were repulsed at Antietam and Gettysburg. The reason Lincoln won where LBJ lost is because Lincoln was able to politically outmaneuver the rebels and their Copperhead allies in the North. Some of the methods Lincoln used to do that were admittedly extreme, but I’m not convinced Johnson would have had to go to the same extremes.

War is hard. Unless you win the entire war in a month or two, it’s always going to be easy to spin things and make it look as if the war is going badly. This problem gets worse as communication gets faster and attention spans get shorter. Vietnam seemed worse than it was because the government wasn’t prepared to fight a war in the age of color television. Britain had similar challenges fighting the Crimean War during the age of the telegraph, and the US struggled to fight the wars in Iraq and Afghanistan during the eras of 24 hour cable news and social media. (Which is not a statement in defense of those wars, but rather a statement about the challenges of operating in a new media environment).