After WWII, American Cold War policy was based on the “domino theory”— if one country fell to communism, the surrounding countries would fall, like dominoes.
The Eisenhower administration was worried that the potential fall of Vietnam could result in communist control of other Southeast Asian and Pacific nations.
In 1955, the Southeast Asia Treaty Organization (SEATO) was formed to prevent communist expansion in the region.
In February 1955, President Eisenhower sent ~700 military personnel plus military and economic aid to the government of South Vietnam.
Source: JFK Library