Just like in Korea, the South was capitalistic while the North was communistic. The foreign policy of US at the time was to keep communism contained and to support any regime that was for capitalism.
weapons
US Military Advisers were in South Vietnam to help the South defend themselves. When that didn't work, US Regulars were sent in. When that didn't work, they vacated the field.
1954 A+
The US helped the SOUTH Vietnamese. The North was the enemy!
The US offered to help south Vietnam in 1961 to help Vietnam preserve its independence from the North. It was during the Cold War, and the US was fighting Communist Russia across the world.
The US didn't invade South Vietnam. The Republic of South Vietnam asked for US help as they couldn't stem the Red Tide (communist infiltration).
Defending South Vietnam against Communist aggression from North Vietnam.
South Vietnam was already a sovereign nation by the time the US entered the nation. Its title was the "Republic of South Vietnam."
Australia mirrored the US. We sent in US Advisors and Special Forces (Green Beret), the Australians did the same (with their equivalent). When the US escalated, the Australians escalated.
N. Vietnam was ATTACKING S. Vietnam. South Vietnam was defending itself with the help of the US/Allies.
RVN (Republic of South Vietnam); which no longer exists.
US sent military aid to France in 1940 to help them take over Vietnam.