"Lana _san" -- (1) is completely inaccurate as the USMC were sent in in April 1965 because the NLF were beating the ARVN on the ground. (2) is also inaccurate. The US were bombing areas in Laos as early as 1961 and were bombing Cambodia (unofficially from 1965) from 1969 onwards (the Breakfast Bombings). (3) The ARVN broke the ceasefire in 1973 and yes, the PAVN and NLF attacked in February 1975 and overrun the rest of southern Vietnam.
++++++++++++++
1) The US had violated the "1954 Geneva Agreements on Indochina" in 1954 as the agreements were being signed and the "Republic of Vietnam" was an illegally created nation. US military advisers were insufficient in training the ARVN troops, and the ARVN troops were being beaten on the battlefield fighting against the "National Liberation Front of Southern Vietnam" (NLF) which had been created in December 1959 at a conference just outside of Saigon.
USMC troops were sent as the "first combat fighting force" in April 1965 and it was three months later that the first "Peoples Army of Vietnam" (PAVN) troops were sent south of the DMZ in combat units to assist the NLF. Prior to that "Southerners" who were members of the PAVN had been sent south to train the NLF.
2) It would depend upon what year you are talking about. In the early days of the war, the political leaders of both parties were very supportive of the war. As the war progressed and the number of "body bags" started to arrive home along with the combat footage one saw on TV, the people started complaining and then the politicians, especially Democrats, started to slowly change to apposing the war.
In 1973 Congress passed several laws that restricted arms supplies to the ARVN in Vietnam, the Lon Nol forces in Cambodia and the Right-wing troops in Laos. They also stopped the US bombing in Cambodia from 15 August 1973. (I covered the bombing halt for UPI/UPITN). These laws were passed because the US embassies and militarty had lied to Congressmen during a "fact finding trips in April 1973).
3) No, neither side really won in 1973. The ceasefire could have led to a peace agreement and a more peaceful end to the war if the ARVN had not violated the ceasefire within hours of it supposedly taking affect.
The US lost face to some degree in 1973 but when the ARVN were completely defeated in 1975 the US really had lost face and therefore broke all their previous agreements on Vietnam negotiated in 1972-73.
4) Many historians, especially US historians, still have/had an "anti-communist" attitude, and therefore felt that the war was worth fighting.
Other historians, mostly non-Americans, considered the war wrong and illegal as the US violated the "1954 Geneva Agreements on Indochina" and also violated international law. The "Republic of South Vietnam" was an illegally created nation (in violation of international law and the Geneva agreements).