We never tried to force the people of Vietnam to be free. We tried to give them a chance to decide by keeping back the totalitarian brutes. And no, I don't think most Vietnamese supported the communists. I know of no evidence for that claim, beyond shallow leftist slogans.
It's very sweet to think of Nepalis, or anyone else, "deciding what form of government they want." But usually, the shape of such things is set by men with guns.
In Nepal, at least, there are men with guns who don't want anything like democracy, and other men with guns who either prefer it or would tolerate it.
On one side, we have communists, who have no intention of letting the Nepalis decide anything for themselves. On the other hand, we have an old-fashioned monarchy that has allowed measures of democracy, pulling back on them because it is under mortal threat. Prospects for your scenario are better if the communists are not only defeated, but defeated very soundly.
Therefore, the morally responsible person, rather than wash his hands of a situation, tries to decide which men with guns are best, or least-bad. A country with interests as vast as the U.S. must often make such decisions in regard to foreign relations.
If this government stays in power, there will be a better chance for freedom than if the communists win.