The Communist North Vietnam acting in support and defence of the capitalist South Vietnamese.
Washington was at war with Hanoi.
North Korea & North Vietnam.
US presidents consistently used the words, "stopping Communist aggression" during their speeches.
The allies during the Vietnam War were the US, South Viets, South Koreans, Thai's, Philippines, Australia and New Zealanders.
about 1957
The USAF & USN fought in NORTH Vietnam (in the skies); the US Army & USMC fought on the ground in SOUTH Vietnam.
Because of US Policy to fight communism.
North Vietnam (capital was Hanoi).
To fight communism.
The US fought for the South.
South Vietnam was considered an ally of the US.
Plenty of US Servicemen of Irish descent fought in the war, but Ireland did not fight in the Vietnam War.