The war 1964-68
Although in the American thinking the Vietnam War is a 15-20-year-long war, the active fight started only in August, 1964 with the Tonkin Gulf incident. They accused North Vietnam of the broke of the Geneva Treaty. Since the independence of the Democratic Republic of Vietnam (DRV) had been accepted by every country, they didn't had the right to spread the communism to South. However, the USA didn't take in consideration the desire of the South Vietnamese people. The North supported the organizations and got bigger and bigger influence in the region. That's why the Great Power couldn't just sit back and watch.
The US protected not only their advisors, but launched the air expedition named Rolling Thunder, which claimed a lot of lives. During the offensive the US-Army lost a lot as well. On 29 April, 1966 according to the North' official report they shot down the 1000th American aircraft.
Meanwhile, South was totally puzzled. In 9 month had 13 cabinet the power. This made only the Vietcong stronger who began to attack already American targets. In December, 1964 they took the village Binh Gai that is only 40 mile far from the capital, Saigon. That's why the USA interfered more actively which resulted the air offensive. This couldn't achieve his aim because the Soviet Prime Minister, Alexei Kosigin, came to Ha Noi and after his visit the North Vietnamese were sent new from the ground starting air defensive rocket-systems (SAM). China signed an agreement too about supporting North technologically and economically.
In 1965 Nguyen Cao Ky got the power in South who continued the repressive politics of Diem which resulted small student and Buddhist movements. The US continuously sent soldiers to the area and other countries, like South Korea, Australia sent units, too. Although they raised the number of the US-fighters from 60000 to 270000, they didn't know the Vietcong, not even their liaisons, so they had no success. They didn't know them so much that they desperately tried to capture them, but it costed just more and more lives. One of their biggest defeats was in Dong Nam Bo, 1967 when they attacked the center of the revolt. They put into action even the Huey Cobra offensive helicopters and had more than half million soldiers in South plus the troops of New Zealand and Thailand. Approximately 700000 soldiers were sent from Australia, South Korea, New Zealand and Thailand. At the end of 1967 the two countries started the peace negotiations, but these failed because of the South's administration's being corrupt