The Vietnam War (1957–1975) was the primary foreign policy challenge for both presidents. Johnson led the United States directly into the war in 1964, with the beginning of air raids, and escalated in 1965 with the entry of American ground troops. Upon becoming president in 1969, Nixon brought both continuity and change to different aspects of American involvement in the war.

First, Nixon continued Johnson's policy of negotiation with North Vietnam. After the shock of the enemy's Tet Offensive in early 1968, Johnson gave up on achieving a military victory. He opened peace talks in mid-1968, but progress in those discussions were elusive.

Nixon also ruled out an American victory on the battlefield. Henry Kissinger, who was Nixon's chief foreign policy adviser, continued peace negotiations with Hanoi. They dragged on for years.

Nixon's approach to the war was both contradictory and complex. Johnson had hoped that America's military forces would win the war. Nixon's Vietnamization strategy was implemented in order to have the South Vietnamese gradually take over the fighting; Nixon slowly withdrew American combat forces from Vietnam. At the same time, Nixon expanded the war by invading Cambodia in 1970 and Laos in 1971. In 1972, he mined Haiphong Harbor.

Both presidents had to confront growing protests in the United States over the Vietnam War. Those protests, which had led to Johnson's decision not to run for reelection, became more intense under Nixon. In 1970, four protesters were shot dead at Kent State University. Domestic opposition to the war intensified Nixon's obsession with secrecy, and helped set the stage for Watergate.