After the fall of Dien Bien Phu in 1954, an international conference in Geneva established that foreign troops would withdraw from Vietnam pending an election in '56 that would unify the country. The Viet Minh withdrew to the north to await the election, but the US refused to participate or to recognize the victor--because we believed that Ho would win. We continued to prop up the Diem regime in the south. But Eisenhower never saw this as a major US commitment--there were only 400 US military advisers when he left office.
JFK boutht into the "domino theory" and saw Vietnam as a big deal--there were 16-18k US troops in Vietnam when he died. Then LBJ got pumped up with the notion that "if we don't fight 'em in the jungles of Vietnam, we'll be fighting 'em on the beaches of California." But even he never had a vision for what victory would look like--at best, he saw Vietnam as an Asian version of the FDR-era Works Progress Administration. Documents and tapes released from his secret Oval Office conversations show him totally baffled by the war and having no clue about how to get out of it. By the time Ford finally pulled the plug, 58,000 Americans died for those mistakes and misjudgments (as have another nearly 4k for W's "weapons of mass destruction"). And so far, no guerilla warfare has broken out on the shores of California.
The war would never have been waged on that scale, for that long, if there hadn't been a draft to provide an endless supply of warm bodies--and the threat of a five year prison term and a $10k fine for those who refused Uncle's invitation.
As for the war being "illegal" or "unconstitutional": The Constitution unambiguously appoints the President as Commander-in-Chief. He has to be able to move the military into action immediately to protect the country and respond to threats or attacks. But setting us out for a protracted military campaign is more ambiguous. Yes, he needs Congressional approval to declare war or a state of national emergency. But it'd take another Pearl Harbor to get Congress to declare war because of the vast extra powers that the President gets. That's why LBJ asked for the Tonkin Gulf Resolution (and W asked for a similar bill before invading Iraq). After LBJ's buildup, Congress also passed a law requiring the President to report intentions (i.e., ask Congressional approval) if a military action lasts longer than 30 days. Constitutional scholars doubt that any such requirements are Constitutionally binding because of the "Commander-in-Chief" clause. But Presidents have chosen to get "sense of Congress" declarations to provide cover for them anyway. If push comes to shove, the only way Congress can stop a war is to refuse to approve the Defense Department's budget--and that'll never happen.
