There are some geographic obstacles to a surviving South Vietnam.
South Korea is protected on three sides by ocean. South Vietnam wasn't. Meaning the North can funnel supplies through Laos and Cambodia, and to remove that threat would open up another front. Even Nixon refused to go more than a few (relatively) miles into Cambodia.
I don't think it's an "unwinnable" war, but then again, the "winnability" of a war really shouldn't factor into whether it was justified or not.
South Korea is protected on three sides by ocean. South Vietnam wasn't. Meaning the North can funnel supplies through Laos and Cambodia, and to remove that threat would open up another front. Even Nixon refused to go more than a few (relatively) miles into Cambodia.
I don't think it's an "unwinnable" war, but then again, the "winnability" of a war really shouldn't factor into whether it was justified or not.