In a nutshell:
Thanks to the USSR's 6 days of support (more or less) in the Pacific Theater of Operations during WWII, the Vietnam War became a continuation of the Korean War against China, which was a continuation of World War II directed against occupying Soviet troops, fed by China. Vietnam was the same thing from a different port of entry, but with fewer Russians. All the UN needed was a glimmer of need from the region so they could mount humane forces against evil China, and they got it, in the form of civil unrest in the southern region of Vietnam directed against the north.

You see, South Vietnam was once a piece of Vietnam, which used to be part of French Indochina. When the bombs were dropped on Hiroshima and Nagasaki, the USSR declared war on Japan (yeah, after the fact) and swept across southern China (at the time, both the USSR and China were Ally members), dropping off "security forces" along the way.

Much later, after Korea had settled down, hostilities were still brewing in Vietnam. At the end of WWII (AKA, the Indochina War), Vietnam managed to break away, fairly destroying the French colonial nation of Indochina, turning now to a north-south conflict. North and South were of feudalistic, dynastic descent, and both deeply based on communistic designs, but the North was accepting rule from evil China, which the South refused (China's influence was the only political difference).

This is all the prompting the UN actually required to "aid" the people who were being "oppressed" by the mighty Red Dragon. The nation was then split in half, in an attempt to repeat the tactics won in Korea, but this failed, as North and South remained communist, desired reunification, and thwarted UN attempts to "change" the southern communistic nation to democracy.

Various persons were put in charge of various regions, many with personal agendas, some with hopes of drug-cartel profit, some even stealing community treasures, all of which did nothing to mend the rift. The small, caring trooper became mere cannon fodder to the pirates in charge.

The rest is history. We went in, did heroic stuff and, at the fall of Saigon (which determined for the US Congress that the South Vietnamese forces were "refusing" to hold their own), we left.

BTW, reunified Vietnam seems to have recovered well. Not perfectly well, but well. There is still a lot of work needed in the area of human rights in the region, but then what nation is really perfect?