It’s in the best interests of the United States and Vietnam to contain the Chinese.
I understand what you are saying, but think this issue is a great argument against ever answering a draft call-up. I don’t want myself or a loved one to die in a war against X and a few years later the government that sent us there “makes peace” with the people we were sent to fight. As treacherous as this deal is, it was even worse when we normalized relations with North Vietnam. There were a lot more vets still alive at that point...