U.S., South Vietnam, and Allies versus North Vietnam and National Liberation Front (Viet Cong).
- President Truman sends 35-man military advisory group to aid French fighting to maintain colonial power in Vietnam.
- After defeat of French at Dien Bien Phu, Geneva Agreements (July) provide for withdrawal of French and Vietminh to either side of demarcation zone (DMZ) pending reunification elections, which are never held. Presidents Eisenhower and Kennedy (from 1954 onward) send civilian advisers and, later, military personnel to train South Vietnamese.
- Communists form National Liberation Front in South.
- U.S. military advisers in South Vietnam rise from 900 to 15,000.
- Ngo Dinh Diem, South Vietnam's premier, slain in coup (Nov. 1).
- North Vietnamese torpedo boats reportedly attack U.S. destroyers in Gulf of Tonkin (Aug. 2). President Johnson orders retaliatory air strikes. Congress approves Gulf of Tonkin resolution (Aug. 7) authorizing president to take “all necessary measures” to win in Vietnam, allowing for the war's expansion.
- U.S. planes begin combat missions over South Vietnam. In June, 23,000 American advisers committed to combat. By end of year over 184,000 U.S. troops in area.
- B-52s bomb DMZ, reportedly used by North Vietnam for entry into South (July 31).
- South Vietnam National Assembly approves election of Nguyen Van Thieu as president (Oct. 21).
- U.S. has almost 525,000 men in Vietnam. In Tet offensive (Jan.–Feb.), Viet Cong guerrillas attack Saigon, Hue, and some provincial capitals. In My Lai massacre, American soldiers kill 300 Vietnamese villagers (March 16). President Johnson orders halt to U.S. bombardment of North Vietnam (Oct. 31). Saigon and N.L.F. join U.S. and North Vietnam in Paris peace talks.
- President Nixon announces Vietnam peace offer (May 14)—begins troop withdrawals (June). Viet Cong forms Provisional Revolutionary Government. U.S. Senate calls for curb on commitments (June 25). Ho Chi Minh, 79, North Vietnam president, dies (Sept. 3); collective leadership chosen. Some 6,000 U.S. troops pulled back from Thailand and 1,000 marines from Vietnam (announced Sept. 30). Massive demonstrations in U.S. protest or support war policies (Oct. 15).
- U.S. troops invade Cambodia in order to destroy North Vietnamese sanctuaries (May 1).
- Congress bars use of combat troops, but not air power, in Laos and Cambodia (Jan. 1). South Vietnamese troops, with U.S. air cover, fail in Laos thrust. Many American ground forces withdrawn from Vietnam combat. New York Times publishes Pentagon papers, classified material on expansion of war (June).
- Nixon responds to North Vietnamese drive across DMZ by ordering mining of North Vietnam ports and heavy bombing of Hanoi-Haiphong area (April 1). Nixon orders “Christmas bombing” of North to get North Vietnamese back to conference table (Dec.).
- President orders halt to offensive operations in North Vietnam (Jan. 15). Representatives of North and South Vietnam, U.S., and N.L.F. sign peace pacts in Paris, ending longest war in U.S. history (Jan. 27). Last American troops departed in their entirety (March 29).
- Both sides accuse each other of frequent violations of cease-fire agreement.
- Full-scale warfare resumes. South Vietnam premier Nguyen Van Thieu resigns (April 21). South Vietnamese government surrenders to North Vietnam; U.S. Marine embassy guards and U.S. civilians and dependents evacuated (April 30). More than 140,000 Vietnamese refugees leave by air and sea, many to settle in U.S. Provisional Revolutionary Government takes control (June 6).
- Election of National Assembly paves way for reunification of North and South.