American involvement in Vietnam increased gradually through a process that in today's terminology would be described as "mission-creep". In the mid-1950s, the US was bankrolling France's colonial struggle against a communist independence movement. By 1968, it had 500,000 troops in the country.
It's ostensible cause, as outlined by President Kennedy in 1956, was to protect "the free world in Southeast Asia" from "the red tide of communism overflowing into Vietnam". Known in the western world as the "Vietnam War" and in Vietnam as the "American War", the conflict was bloodthirsty by anybody's standards. Over 1.1 million communist (North Vietnamese and Viet Cong), 223,000 South Vietnamese and 58,000 US soldiers lost their lives.
Richard Nixon's protestations in his 1985 book No More Vietnams that "no event in American history is more misunderstood than the Vietnam War", is certainly the marginal assessment of the conflict. More common is the line taken by Dr Leslie Kennedy in America's Wars in Asia in which she contends "by most accounts, the Vietnam war was a disaster -- a military disaster, a political disaster, an economic disaster, an ecological disaster, and a personal disaster for many of the people involved".
The war's legacy has been enduring. Vietnam shattered the illusion of American invincibility and has become a universally understood analogy of misguided military intervention. The phrase "another Vietnam" is now routinely used to contemptibly describe conflicts like Iraq or Afghanistan by those who believe that victory for the west is unattainable.