When push came to shove China supplied the troops and materiel to drive the UN (US) back to the 38th parallel, but no farther. With shorter supply lines and greater manpower, there is no question they could have driven much farther had they wanted to pay the price.

The US couldn't have driven back north at any price. Too many of 'em.

I suggest you start your reading with a more detailed account of the Korean War, including where the front lines were near the end of 1950.

Without the U.N. (read: Mostly the U.S.) the communists would have eaten the entire peninsula, and what is now the prosperous nation of South Korea would have been the bark-eating paradise North Korea is now.

The U.S. maintained permanent forces in Korea for half a century after that. I think there were still almost 40,000 troops based there when Bush took office, or almost a third of the number in Iraq. As a comparison, I think there are still about 70,000 U.S. troops in Europe.

I would have had no problem with setting up a similar line of defense in Vietnam along the border, with permanent bases. Does that answer your question, or are you going to twirl off in yet another direction?