We had OSS there during WWII and we were friendly with Ho Chi Minh (he provided intel on the Japanese and helped in finding downed US pilots). Ho wanted US assistance in gaining full independence from the French.
Truman, Eisenhower, Kennedy sent 'advisers' to assist the Republic of Vietnam against Ho Chi Minh and the north in the late 1940s and throughout the 1950s and into the early 1960s. The US provided about 80% of the supplies the French were using in the war against Ho Chi Minh.
Johnson escalated it on a huge scale in 1965-66.
Truman and the US freaked out after China fell to the communists in 1949. Suddenly, Vietnam was important.
Red Scare begins.
Where The Domino Fell: America and Vietnam 1945-2006, James Olsen & Randy Roberts