The Vietnam War, America's longest war, had ended up in failure for the US. At that time, and until today, the US was the strongest nation on the planet and Vietnam was one of the poorest and a primary industrial country, where peasantry was in majority. The war, more officially, began from 1964 and ended in 1972 and the main reasons for losing the war was underestimation of VC, Ho Chin Minh Trail, Vietnamese people, protests back in the US, efficiency of US troops, cultural differences, and US casualties.
The US has underestimated Vietcong and the NLF. They had supply from the USSR and China, such as MiG fighters that shoot down more than 700 airplanes and the leader of VC, Ho Chi Minh, saw how Mao used the guerrilla tactics. In early 1968, the Tet offensive has proven that 'No Where', 'Nothing', 'No one' was safe in South Vietnam at 'Anytime'.
It came to that Saigon, capital city of South Vietnam where had most of the US bases and the US embassy, had been attacked by Vietcong. Obviously the US had underestimated them. Nonetheless, they were not trained to fight in jungles against guerrillas, even Green Berets were not practical in this environment, thus they, militarily, had not much advantages. Not only the Ho Chi Minh Trail but Vietcong also had Cu Chi tunnel system which US troops never figured out.
North Vietnamese, and the local people supplied needs and ordnance to Vietcong, mainly from North Vietnam, through Ho Chi Minh trail. US airplanes have never stopped try bombing this trail but they've never succeeded and figure out where they are. They were not through high ways or big roads but through farmlands which could hardly been seen on airplanes. Also, it covered Cambodia, which made decisions even hard...