The US was bankrolling the French war there and after the French left following their defat at Dien Bien Phu, the US started to put money into Ngo Dinh Diem. He wasn't really all that popular with the people of the south, but the billions he received (as part of the US promulgation of the Truman Doctrine (help the free peoples of the world) allowed him to build an army to fight the 'troops' of Ho Chi Minh--and they needed those troops, because when the US prevented free elections (they knew the Communists would win) then the war really started. Operation Phoenix (?) led to the entrenchment of American 'advisors'--CIA, military. The north was impossible to stop. The US then got morassed in a ground war because despite dropping more explosives on that tiny little country than was dropped by them in WWII, there was no way to prevent infiltration by NVA regulars or VC irregulars. So from the tunnels of Cu Chi to the HCM Trail, American kids went to help a people who really din't want that type of help. IMO, the capitalists did, but not the average guy on the street.

(I had intended to do a Masters on that war, but life intervened.) IMO, the best book about it all is Stanley Karnow's "Vietnam: A History". Karnow received a Pulitzer for his work as a btw.