The USSR gave aid to North Vietnam. China sent aid to the Vietcong via North Vietnam.
North Vietnam started the war against South Vietnam; the US went to the aid of the South.
which war
US Military advisers in '55.
President Eisenhower sent military advisors and maybe some technicians to Vietnam to help the government there stay in power. That was the beginning of US involvement there.
Mrs young might know
France
Feared the spread of communism in Southeast Asia
This is not a complete authoritative answer but I do know that during this time the spread of communism was a great concern and priority for the US government and to some degree our allies. North Vietnam's ruler Ho Chi Minh I believe was dedicated to the cause of socialism and wanted to spread this governing ideology to south Vietnam. It was considered to be in the best interests of the US to help prevent the spread of communism to the South.
The USSR gave aid to North Vietnam. China sent aid to the Vietcong via North Vietnam.
The US supplied massive military aid to South Vietnam. All in vain, as it turned out.
North Vietnam started the war against South Vietnam; the US went to the aid of the South.
US sent military aid to France in 1940 to help them take over Vietnam.
which war
They feared that Vietnam would become Communist if it should gain indepedence.
See website: Genocide
Nicaragua