The Indochina War can be divided into the following phases:
In the second phase, which is the war between North and South Vietnam, US financial aid was for South Vietnam. North Vietnam was communist, and many landlords fled to South Vietnam. The North Vietnamese wanted a united Vietnam under the Communist banner, but South Vietnam was not only aided by US, the ruler was also a US-backed personnel, Diem. President Diem of South Vietnam was assassinated in 1963. Meanwhile, South Vietnamese communists, known as the Viet Cong (the North Vietnamese were the PAVN) were fighting the South Vietnamese ARVN (the South Vietnamese army in support of Diem).
US involvement began by a ploy. The Tonkin Gulf Incident was claimed to have happened, and attests that Vietnamese boats fired at American warships in international waters. It was later discovered that months before the Tonkin Gulf Incident, there was already a Tonkin Gulf Resolution, which was a plan for an American war in Vietnam. Hence began American involvement, as the US sent troops into South Vietnam. The US troops fought by the side of the ARVN, but the deciding factor that allowed the Viet Cong, whose ranks included only 60,000 Viet Minh, to last against the combined army five times its strength was that the US troops were unacquainted with guerilla warfare, and hence could not boost the fighting capabilities of the South Vietnamese army much.
The Americans have also been criticised for the My Lai massacre, though the Vietnamese committed their fair share of atrocities as well. Aerial bombardment of the Ho Chi Minh trail, the main supply line from North Vietnam to the Viet Cong, proved fruitless. American death tolls ran as high as 273 per week at the height of the third phase of the Indochina War. Perhaps the most well-remembered incident would be the Tet Offensive. On the day of the Vietnamese New Year, the Tet, Viet Cong attacked 36 of 44 South Vietnam cities. Their main aim was to spark a national uprising for a united Vietnamese cause and to expel the Americans. They did not succeed in this aim, but they proved that they were capable of urban warfare as well. Of course, the Viet Cong suffered heavy casualties and were effectively wiped out.
By 1973, domestic pressure have been heavy and the Americans withdrew their forces. Notable was Nixon's "Vietnamization", that is, handing the fighting over to the South Vietnamese ARVN while withdrawing US troops batch by batch. American withdrawal was almost complete by 1973.
With the departure of the Americans, South Vietnam swiftly fell under the Viet Cong and North Vietnam. Saigon was overrun by 1975 and Vietnam was hence united.