In order to fully understand the reasons as to why the US lost its war against Vietnam, one must fully understand the events and key factors before and during the war itself. The war itself took place between 1961 and 1975, resulting in US defeat. Vietnam had been an independent nation until the French conquered the country in 1887, renaming it French Indochina. Throughout the years of the French rule, Ho Chi Minh, a Vietnamese nationalist, formed the Viet Minh communist party. The aims he had was to free Vietnam and advocate for its independence from the French. In 1945, Japan moved into Vietnam and fight to end the colonial era and declare Laos, Cambodia and Vietnam, who are all under French rule, as independent. 5 months later they encounter defeat, and the French began to reassert their authority throughout Vietnam. Ho Chi Minh declared North Vietnam independent at the end of 1945, which resulted in the US supplying military assistance to France to combat the communist expansion throughout the North Vietnam. After the French defeat, in 1954, their rule in Indochina officially came to an end, however the US decided to remain. The US was based mainly in the South of the country, so the communist forces in the north created a supply trail called the Ho Chi Minh trail through Laos and Cambodia to supply guerrilla attacks in the south against the government.
In 1962, US president John F. Kennedy ordered US aircrafts to spray Agent Orange to kill vegetation in order to cut off food for the Ho Chi Minh Trail, however the effects this had on civilians were inhumane and resulted in a great loss of support for the US. After JFK's assassination, Lyndon B Johnson becomes the president of the United States, and immediately called for air strikes on North Vietnam, commencing years to come of intense and aggressive attacks. By 1967 there were 500,000 US troops in Vietnam. This same year, protests against US involvement and against the war itself start happening around the world. After widespread protests and an overwhelming show of dissatisfaction with the way the war was progressing, President Johnson, decided to not run for re-election, and Nixon is elected president. Nixon's administration progressively reduced troops in Vietnam and Vietnamization begins. North Vietnam began gaining control of the South slowly pushing the US further out into Cambodia, and in 1973 Nixon signed the Paris Peace Conference ending US involvement in the Vietnam War. This was effectively a surrender as the US Embassy staff along with as many civilians as possible were desperately evacuated from Saigon as the country (and Embassy) were overwhelmed by the People's Army of Vietnam.
The war was long and full of events and key happenings, which allowed space for a lot of failure. There are many reasons for US failure: underestimating Vietnam, inability to sustain big losses, failure to generate and maintain support from the American public and worldwide, and so on.
Towards the beginning of the war, American public were mostly in favour of intervention and sending in more troops. This support carried out until live coverage of the war was released, and people became extremely disturbed by what they were seeing, and in 1971 records show a record low level of confidence and support. The main factor behind their loss is support is the inhumanity and aggression the US used to fight the Vietnamese.
The US veterans were blinded by their hatred towards the Viet Cong to the extent that they would massacre whole villages, treat civilians that they were meant to be helping bad etc. The most famous of massacres was the My Lai on the 16th March 1968, in which the Americans beat and killed 347 unarmed civilians. This same year the CIA introduced a system called the Operation Phoenix in which they captured and interrogated tens of thousands of Viet Cong supporters, most of which never made it out alive. A US marine stated that their emotions were very low because we'd lost a lot of friends which he used as an excuse for when they entered a village full of suspected VC supporters, they gave it to them whatever was moving was no longer moving anymore. Among many, many massacres, another key form of attack that lost the US a lot of support was the release of the Agent Orange gas. They released 73 million litres worth, which was in theory was to kill of vegetation and cut off supplies to the guerrilla war fighters, however the effect this defoliant had on the people of Vietnam tortures them to this day. Lakes, soil and food chains have all been irreversibly damaged, people have been suffering from serious birth defects such as spina bifida, deformed limbs and so on. These methods used by the US aliwnated friendly and neutral Vietnamese and americans themselves, contributing massively to their failure in Vietnam.
America had a lot more military power, access to high-tech power and more finance available to them to fights, however they lacked knowledge of the land they were fighting on. The Vietnamese took this to their advantage and organised guerrilla movements to attack American troops unexpectedly, as well as creating the Ho Chi Minh Trail to be able to transport food and supplies to guerrilla war fighters in the south, while in the meantime move wounded soldiers and civilians up north. This train created a vital advantage to the communists. It wasn't a single route either, it spread out and divided into multiple routes so that in the case of a US bombing, they wouldn't be haltered, and they would be able to continue their path another way and maintain the movement of supplies. Although the bombs released on this trail resulted in $6000 trucks, America in fact lost a lot more, they lost many million-dollar bombers which were far harder to replace. Moreover, the communists had tunnels they would booby trap in case the Americans managed to get in. There was a case in which a US marine was killed by a booby trap just a few miles from VC headquarters, but the rest of the troop managed to get out before they were killed. 25% of American wounded soldiers were victims of booby traps rather that direct enemy attack. In addition to these tunnels, they created Shock Brigades in the case of bombings or air attacks, a siren would go off and around 2 million northerners could disappear immediately and stay safe from the attacks.
The American military strategy lacked efficiency too. They fought in a search and destroy manner, however as stated before, the Vietnamese were experts in hiding, be that in underground tunnels, or in their guerrilla positions. A CIA report in 1967 stated that only 1% in nearly 2 million unit operations resulted in enemy contact.
President Nixon believed that the American soldiers were too soft to fight the communists. When the US left Vietnam there were basketball courts installed, volleyball fields, swimming pools etc. He stated that If we fail it will be because the American way simply isn't as effective as the communist way I have an uneasy feeling that this may be the case we soften them up rather than harden them up for battle. A fighter in the PAVN (People's Army of Vietnam), expressed his thoughts on the difference of both sides: You ask me what I thought of the American it was difficult for them to suffer all the hardships of the Vietnamese battlefront. When we had no water to drink, they had water for showers! We could probably suffer the hardship much better than they could. That probably was the main reason we won.It is clear that both the US itself, and the Vietnamese both believed the US marines weren't hard enough to win the war despite all their weapons. American soldiers were so frustrated that their disciple slowly deteriorated: about a quarter caught STD's, 22% shot up heroin, and 58% smoked marijuana.
The war had many effects on the US society for years to come. They suffered great economic losses for starters, between 1965 and 1973, $15.5 billion dollars was spent on their people, whereas they spent $120 billion on the Vietnam war. Moreover, American veterans who returned from the war suffered very severely both mentally and physically, became drug addicted, suffered severely from PTSD, and were treated horribly by their own people when they returned from war. The united states also lost a great amount of confidence in their military abilities considering how such a great superpower like them lost to a much smaller and less powerful power such as Vietnam.