The USA was not able to defeat the Vietcong (any Vietnamese person supporting communism and the North) for a number of reasons:
The American tactics turned the Vietnamese people against them.
The Americans used chemical weapons Napalm (jellied petrol) and Agent Orange (superior strength weed killer). This was used to clear foliage in the jungle which was the natural hiding place for the Vietcong. They also wanted to see along the Ho Chi Minh Trail, the Vietcong’s supply route.
Napalm did clear much of the undergrowth but it also stuck to humans and caused horrific injuries. Agent Orange also cleared the foliage but many innocent civilians’ farms and crops were lost, and animals were killed.
To prevent the Vietcong from hiding in Southern villages, the South Vietnamese peasants were taken from their villages and settled in ‘strategic hamlets’. These were surrounded by barbed wire and controlled by the Americans.
The Americans bombed strategic targets in North Vietnam to stop the supply of troops and weapons to the South. Bombs often missed targets and hit schools and hospitals.
The Americans tried to find Vietcong who were hiding in villages by dropping in by helicopter and killing those they thought were Vietcong.
In 1968, a mission to the village of My Lai ended with the massacre of many innocent civilians, including women and children.