What impacts did the Vietnam War have on United States foreign policy?

Before withdrawing from Saigon (the capital of South Vietnam) in 1975, the United States had never lost a war. US involvement in Vietnam really began after World War II. During the war, Japan invaded Vietnam and kicked out the French. The French had colonized Vietnam (it was called Indochina at the time), and after the Japanese were defeated at the end of World War II, the French expected to regain control of their former colony. However, the Vietnamese, led by Ho Chi Minh, fought the French for independence from 1945–1954. The US had supported Ho Chi Minh as he fought the Japanese during World War II, but they were hesitant to back him after the war, as the US was also allied with France. The US therefore attempted to stay out of the conflict, although by 1950 the US government was providing financial aid to the French. When the Vietnamese defeated the French at Dien Bien Phu in 1954, the US helped negotiate the peace talks at the Geneva Conference after. At this conference, Vietnam was temporarily divided at the seventeenth parallel. North Vietnam was Communist, while South Vietnam was anti-communist, and free elections were to be held in 1956. However, the US, fearing that Ho Chi Minh would win these elections, backed South Vietnamese President Diem in his refusal to allow free elections to take place, instead allowing Diem to continue as the leader of South Vietnam. Communist guerrilla forces, called the Viet Minh, then began fighting the South Vietnamese in order to unify the country into one communist nation.
At this point, the US began increasing its role in Vietnam. Initially, the US sent 2,000 military advisors to assist the South Vietnamese. Under President Kennedy, the number of advisors increased to 18,000 by 1963. That same year, Kennedy sanctioned a South Vietnamese coup that overthrew Diem, fearing that he might enter into an agreement with the North Vietnamese to establish a coalition government.
Some of the most drastic changes in US policy toward Vietnam occurred under President Johnson (LBJ), who became president after Kennedy's assassination in 1963. Johnson was reluctant to deepen US involvement in Vietnam. However, he also faced political pressure to not appear "weak" and "lose" a country to Communism. The policy of containment, not allowing Communism to spread, was deeply entrenched in US foreign policy goals by this point, and during Eisenhower's presidency, the "domino theory" became popular. This was the idea that if one country fell to Communism, the surrounding countries (in this case Laos and Cambodia) would also be taken over by communists. This fear greatly shaped US policy and action in Vietnam and other countries seemingly threatened by Communism.
Despite being elected in 1964 on a campaign that included a promise not to commit US troops to Vietnam, the first US troops were sent to Vietnam under LBJ. In 1964, the Gulf of Tonkin incident sparked controversy. US intelligence reported that a US destroyer, the Maddox, had been fired on twice by the North Vietnamese. The Maddox, however, was on a secret intelligence mission in North Vietnamese waters, and it remains unlikely that one of the attacks even occurred. Nevertheless, Congress approved the Gulf of Tonkin Resolution, granting LBJ the power to "take all necessary measures" to fight communist forces. This resolution greatly increased the power of the presidency, essentially allowing the president to send troops without an official declaration of war from Congress.
LBJ responded by sending combat troops to Vietnam and by starting Operation Rolling Thunder, a heavy bombing campaign of North Vietnam. By 1967, there were 549,000 US troops in Vietnam. However, the North Vietnamese and Viet Cong had several advantages. First, they were fighting in their own country and had an extensive knowledge of the terrain and jungles. They also utilized the Ho Chi Minh Trail, a supply line that ran through Laos and Cambodia, to move troops and supplies between North and South Vietnam. The North Vietnamese easily replenished their troops and found some sympathy among South Vietnamese people, further complicating the US war effort—if some of the South Vietnamese supported the North, then why was the US even fighting?
US public support for the war, which was already waning, hit a low in 1968 after the Tet Offensive. Prior to the Tet Offensive, US General Westmoreland claimed that victory was in sight in Vietnam. Then, the North Vietnamese and Viet Cong launched a surprise attack, surrounding the US embassy in Saigon and overrunning cities throughout South Vietnam. Although the North Vietnamese and Viet Cong were eventually driven back, the Tet Offensive was a huge embarrassment for the US and signaled that the US had been misleading the public about the war. The war was looking increasingly unwinnable, and LBJ chose not to seek reelection in 1968. He also called for the beginning of negotiations with North Vietnam.
However, the end of Johnson's presidency and the election of Richard Nixon in 1968 did not signal the end of war. Nixon promised "peace with honor" in Vietnam during his campaign, meaning he would seek a way to end the war while preserving America's integrity as a world power. A big part of Nixon's plan was "Vietnamization"—the gradual withdrawal of US troops and ending of the draft in order to turn the fighting of the war over to the South Vietnamese. At the same time, Nixon invaded Cambodia and then increased secret US bombings of both Cambodia and Laos in order to cut off the Ho Chi Minh Trail. Ultimately these actions had the effect of further angering the American public and destabilizing Cambodia, allowing the oppressive Khmer Rouge to come to power. Finally, in 1973, Nixon agreed to a cease-fire and withdrawal from Vietnam. In 1975, the last US troops left Saigon, and the South Vietnamese soon fell to North Vietnamese forces.
By the end of the war, the US government had spent $176 billion in Vietnam. Over 58,000 Americans died in the war, and the war left a lasting impact on US culture and society. In addition to protests, numerous songs, movies, and novels emerged abut the war. Overall, numerous factors explain the US involvement in Vietnam. The US was perhaps overconfident in its military capabilities after World War II and increasing saw itself as the "policeman" of the world. The Cold War tensions with the Soviet Union increased fears about the spread of Communism and caused the US to see any communist-like government as a pawn of the Soviet Union. Finally, many US policy makers also misunderstood the desires, goals, and commitments of both the North and South Vietnamese.
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