Antigovernment guerrillas began operating in South Vietnam in 1957. Late in 1960 a Communist-led military force known as the Vietcong was organized for the "liberation of South Vietnam." The tempo of fighting increased, as did United States aid to the Diem government. Meanwhile, political opposition to Diem was being ruthlessly suppressed. In 1963 the government was overthrown in a military coup, during which Diem was assassinated.

The generals who took control were unable to achieve political stability. Military regimes rose and fell in rapid succession, and the effort against the Vietcong suffered. With increasing evidence of North Vietnamese assistance to the Vietcong, United States personnel began taking an active part in the fighting. Soon large numbers of American troops were involved. The conflict developed into a full-scale war. In 1968 the United States met in Paris with North Vietnam to begin peace negotiations, joined in 1969 by South Vietnam and the Vietcong. ( .)

Some stability was achieved during the regime of Nguyen Cao Ky, commander of the air force, who was installed as premier in 1965. A constitutional government was established with elections in 1967. Nguyen Van Thieu became president.

Meanwhile, North Vietnam's Ho Chi Minh died in 1969. Throughout the 1960's and early 1970's, the war severely hampered economic development in both North and South Vietnam. In 1972, North Vietnamese forces invaded the south and occupied some territory. At the negotiations in Paris, a cease-fire agreement was signed in 1973, and the United States withdrew from the war. A new North Vietnamese offensive in 1975 resulted in the conquest of South Vietnam. With the end of the war, economic reconstruction was begun. In 1976 the country was formally unified, and in 1978 most private enterprise in former South Vietnam was abolished. During the late 1970's, the totalitarian rule of the North Vietnamese and severe economic difficulties caused many thousands of Vietnamese to flee the country. They became refugees in France, the United States, and other countries.

Border warfare with Cambodia erupted repeatedly during the 1970's. In 1979 the Vietnamese invaded Cambodia and installed a regime friendly to Vietnam. Vietnamese troops supporting the regime remained in Cambodia for a decade afterwards. China, which had backed the ousted regime, invaded Vietnam, but soon withdrew. Government hostility against Vietnam's Chinese minority resulted in some 400,000 Chinese fleeing the country during the 1980's.

In 1995, Vietnam and the United States established diplomatic relations. The first U.S. ambassador to Hanoi was sent to Vietnam in 1997, and in 2000 Bill Clinton became the first U.S. President to visit Hanoi.