No one
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.
US was a primary combatant.
The US asked for their help.
RVN (Republic of South Vietnam); which no longer exists.
Because the war was to help a different country
Guerrilla warfare was largely used by the Viet Cong and not by the US Combatants in Vietnam. To answer your question, no.
austraila and france
We were at war with Vietnam because they disagreed with us about something.
The communist superpowers supported the north with war material. The US supported the south with men & material. Reason: Cold War.
The US entered because they signed a document saying that the will help with a war with communism.
Vietnam
The US is not at war Iraq. The US was at war with a nation called North Vietnam.