Lyndon Baines Johnson (; August 27, 1908 January 22, 1973), often referred to by his initials LBJ, was an American politician who served as the 36th president of the United States from 1963 to 1969. He had previously served as the 37th vice president from 1961 to 1963 under President John F. Kennedy, and was sworn in shortly after Kennedy's assassination. A Democrat from Texas, Johnson also served as a U.S. representative, U.S. senator and the Senate's majority leader. He holds the distinction of being one of the few presidents who served in all elected offices at the federal level.

Born in a farmhouse in Stonewall, Texas, to a local political family, Johnson worked as a high school teacher and a congressional aide before winning election to the U.S. House of Representatives in 1937. He won election to the United States Senate in 1948 after a narrow and controversial victory in the Democratic Party's primary. He was appointed to the position of Senate Majority Whip in 1951. He became the Senate Democratic leader in 1953 and majority leader in 1954. In 1960 Johnson ran for the Democratic nomination for president. Ultimately, Senator Kennedy bested Johnson and his other rivals for the nomination, then surprised many by offering to make Johnson his vice presidential running mate. The Kennedy-Johnson ticket won in the 1960 presidential election. Vice President Johnson assumed the presidency on November 22, 1963, after President Kennedy was assassinated. The following year Johnson was elected to the presidency when he won in a landslide against Arizona Senator Barry Goldwater, receiving 61.1% of the popular vote in the 1964 presidential election, the largest share won by any presidential candidate since the 1820 election.

Johnson's domestic policy was aimed at expanding civil rights, public broadcasting, access to healthcare, aid to education and the arts, urban and rural development, and public services. In 1964 Johnson coined the term the "Great Society" to describe these efforts. In addition, he sought to create better living conditions for low-income Americans by spearheading a campaign unofficially called the "War on Poverty". As part of these efforts, Johnson signed the Social Security Amendments of 1965, which resulted in the creation of Medicare and Medicaid. Johnson followed his predecessor's actions in bolstering NASA and made the Apollo Program a national priority. He enacted the Higher Education Act of 1965 which established federally insured student loans. Johnson signed the Immigration and Nationality Act of 1965 which laid the groundwork for U.S. immigration policy today. Johnson's opinion on the issue of civil rights put him at odds with other white, southern Democrats. His civil rights legacy was shaped by signing the Civil Rights Act of 1964, the Voting Rights Act of 1965, and the Civil Rights Act of 1968. During his presidency, the American political landscape transformed significantly, as white southerners who were once staunch Democrats began moving to the Republican Party and black voters began moving to the Democratic Party. Because of his domestic agenda, Johnson's presidency marked the peak of modern liberalism in the United States.Johnson's presidency took place during the Cold War and thus he prioritized halting the expansion of communism. Prior to his presidency, the U.S. was already involved in the Vietnam War, supporting South Vietnam against the communist North. Following a naval skirmish in 1964 between the United States and North Vietnam, Congress passed the Gulf of Tonkin Resolution, which granted Johnson the power to launch a full-scale military intervention in South East Asia. The number of American military personnel in Vietnam increased dramatically, and casualties soared among U.S. soldiers and Vietnamese civilians. Johnson also expanded military operations in neighboring Laos to destroy North Vietnamese supply lines. In 1968, the communist Tet Offensive inflamed the anti-war movement, especially among draft-age students on university campuses, and public opinion turned against America's involvement in the war.

At home, Johnson faced further troubles with race riots in major cities and increasing crime rates. His political opponents seized the opportunity and raised demands for "law and order" policies. Johnson began his presidency with near-universal support, but his approval declined throughout his presidency as the public became frustrated with both the Vietnam War and domestic unrest. Johnson initially sought to run for re-election; however, following disappointing results in the New Hampshire primary he withdrew his candidacy. The war was a major election issue and the 1968 presidential election saw Republican candidate Richard Nixon defeat Johnson's vice president Hubert Humphrey. At the end of his presidency in 1969, Johnson returned to his Texas ranch, published his memoirs, and in other respects kept a low profile until he died of a heart attack in 1973.

Johnson is one of the most controversial presidents in American history; public opinion and scholastic assessment of his legacy have continuously evolved since his death. Historians and scholars rank Johnson in the upper tier because of his accomplishments regarding domestic policy. His administration passed many major laws that made substantial advancements in civil rights, health care, welfare, and education. Conversely, Johnson is strongly criticized for his foreign policy, namely escalating American involvement in the Vietnam War.

The Vietnam War (Vietnamese: Chiến tranh Việt Nam), also known as the Second Indochina War, was a conflict in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. It was the second of the Indochina Wars and was officially fought between North Vietnam and South Vietnam. North Vietnam was supported by the Soviet Union, China, and other communist allies; South Vietnam was supported by the United States and other anti-communist allies. The war is widely considered to be a Cold War-era proxy war. It lasted almost 20 years, with direct U.S. involvement ending in 1973. The conflict also spilled over into neighboring states, exacerbating the Laotian Civil War and the Cambodian Civil War, which ended with all three countries becoming communist states by 1975.

The conflict emerged from the First Indochina War between the French colonial government and a left-wing revolutionary movement, the Viet Minh. After the French military withdrawal from Indochina in 1954, the U.S. assumed financial and military support for the South Vietnamese state. The Việt Cộng (VC), a South Vietnamese common front under the direction of North Vietnam, initiated a guerrilla war in the south. North Vietnam had also invaded Laos in 1958 in support of insurgents, establishing the Ho Chi Minh Trail to supply and reinforce the Việt Cộng.: 16  By 1963, the North Vietnamese had sent 40,000 soldiers to fight in the south.: 16  U.S. involvement escalated under President John F. Kennedy through the MAAG program, from just under a thousand military advisors in 1959 to 23,000 in 1964.: 131 In the Gulf of Tonkin incident in August 1964, a U.S. destroyer clashed with North Vietnamese fast attack craft. In response, the U.S. Congress passed the Gulf of Tonkin Resolution and gave President Lyndon B. Johnson broad authority to increase U.S. military presence in Vietnam. Johnson ordered the deployment of combat units for the first time and increased troop levels to 184,000. The People's Army of Vietnam (PAVN) (also known as the North Vietnamese Army or NVA) engaged in more conventional warfare with U.S. and South Vietnamese forces (Army of the Republic of Vietnam (ARVN)). Despite little progress, the U.S. continued a significant build-up of forces. U.S. and South Vietnam forces relied on air superiority and overwhelming firepower to conduct search and destroy operations, involving ground forces, artillery, and airstrikes. The U.S. also conducted a large-scale strategic bombing campaign against North Vietnam.: 371–4 The communist Tet Offensive throughout 1968 caused U.S. domestic support for the war to fade. The VC sustained heavy losses during the Offensive and subsequent U.S.-ARVN operations.: 481  The CIA's Phoenix Program further degraded the VC's membership and capabilities. By the end of the year, the VC insurgents held almost no territory in South Vietnam, and their recruitment dropped by over 80%, signifying a drastic reduction in guerrilla operations, necessitating increased use of PAVN regular soldiers from the north. In 1969, North Vietnam declared a Provisional Revolutionary Government (the PRG) in the south to give the reduced VC a more international stature, but from then on, they were sidelined as PAVN forces began more conventional combined arms warfare. By 1970, over 70% of communist troops in the south were northerners, and southern-dominated VC units no longer existed. Operations crossed national borders: North Vietnam used Laos as a supply route early on, while Cambodia was also used starting in 1967; the U.S. bombed the Laotian route starting in 1964, and the Cambodian route in 1969. The deposing of the monarch Norodom Sihanouk by the Cambodian National Assembly resulted in a PAVN invasion of the country at the request of the Khmer Rouge, escalating the Cambodian Civil War and resulting in a U.S.-ARVN counter-invasion.

In 1969, following the election of U.S. President Richard Nixon, a policy of "Vietnamization" began, which saw the conflict fought by an expanded ARVN, with U.S. forces sidelined and increasingly demoralized by domestic opposition and reduced recruitment. U.S. ground forces had largely withdrawn by early 1972 and support was limited to air support, artillery support, advisers, and materiel shipments. The ARVN, with U.S. support, stopped the first and largest mechanized PAVN offensive during the Easter Offensive of 1972. The offensive failed to subdue South Vietnam, but the ARVN itself failed to recapture all lost territory, leaving its military situation difficult. The Paris Peace Accords of January 1973 saw all U.S. forces withdrawn; the Case–Church Amendment, passed by the U.S. Congress on 15 August 1973, officially ended direct U.S. military involvement.: 457  The Peace Accords were broken almost immediately, and fighting continued for two more years. Phnom Penh fell to the Khmer Rouge on 17 April 1975, while the 1975 Spring Offensive saw the Fall of Saigon by the PAVN on 30 April; this marked the end of the war, and North and South Vietnam were reunified the following year.

By 1970, the ARVN was the world's fourth largest army, and the PAVN was not far behind with approximately one million regular soldiers.: 770  The war exacted an enormous human cost: estimates of the number of Vietnamese soldiers and civilians killed range from 966,000 to 3 million. Some 275,000–310,000 Cambodians, 20,000–62,000 Laotians, and 58,220 U.S. service members also died in the conflict, and a further 1,626 remain missing in action.The Sino-Soviet split re-emerged following the lull during the Vietnam War. Conflict between North Vietnam and its Cambodian allies in the Royal Government of the National Union of Kampuchea, and the newly formed Democratic Kampuchea began almost immediately in a series of border raids by the Khmer Rouge, eventually escalating into the Cambodian–Vietnamese War. Chinese forces directly invaded Vietnam in the Sino-Vietnamese War, with subsequent border conflicts lasting until 1991. The unified Vietnam fought insurgencies in all three countries. The end of the war and resumption of the Third Indochina War would precipitate the Vietnamese boat people and the larger Indochina refugee crisis, which saw millions of refugees leave Indochina (mainly southern Vietnam), an estimated 250,000 of whom perished at sea. Within the U.S., the war gave rise to what was referred to as Vietnam Syndrome, a public aversion to American overseas military involvements, which together with the Watergate scandal contributed to the crisis of confidence that affected America throughout the 1970s.