The United States had the most formidable military power in the world, and yet was defeated during the Vietnam War. One battle that was fought between January and April of 1968- the Battle of Khe Sanh-, when the anti-war and anti-draft activities were gearing up in the homefront as people became weary of the high cost of the war, both in material and human terms. President Lyndon Johnson and General Westmoreland tried their best to maintain the base at Khe Sanh, but encountered several difficulties in their attempt to preserve the base. Based on this battle we can see why the Americans were bound to lose
(1) There was no clear territorial objective during the Vietnam War. The Americans were simply told to occupy a bunch of hills, set up a few bases, dig in, wait for air support to come in, and hope that this will wear down the NVA (North Vietnamese Army)
(2) The US had total superiority in military technology and superabundance in terms of munitions, hardware, tanks, artillery and air support. That made American troops very spoilt, as they did not want to do the close-up battle that would have been required in this bushy and tightly filled forest environment. B-52 and F-16 bombers and jets unleashed their devastating napalm over and over again, especially when the US ground troops were besieged by heavy infantry attacks of the North Vietnamese. (The movie “We Were Soldiers” starring Mel Gibson is quite instructive on that front.)
(3) The NVA on the other hand was virtually operating on home ground. They smuggled their forces into South Vietnam thanks to their elaborate tunnel system (the Ho Chi Minh trail) that reaches into their allied countries, Laos and Cambodia. They knew the area very well, and could plan out their pinch attacks on American positions, albeit at heavy loss due to inferior technology. It was next to impossible to take out the tunnel system, and in the later years, Nixon did bomb Cambodia, but that was more of a symbolic move. So was his bombing of Hanoi to expedite the peace treaty with North Vietnam.
(4) One could argue that if the Americans had carried the war into North Vietnamese territory, they would have been able to occupy Hanoi and get rid of the communist government, but that was never part of the deal. The memory of North Korea and the Chinese counteroffensive during the Korean War was deeply imbued in the memory of the American strategists, and another Chinese intervention had to be prevented by all means. It had to be a defensive war of South Vietnam, and that freed up the potential of the NVA to commit all their forces in enemy territory in the South, while constantly receiving re-supply from the homefront (and Soviet Union, China).
And here is the link to the 1-hour documentary. Enjoy watching.