When did the U.S. get heavily involved?

The United States became heavily involved in the Vietnam War in 1965, the year President Lyndon B. Johnson committed large numbers of American combat troops and began sustained bombing of North Vietnam. The shift followed years of gradually expanding involvement that began under President Dwight D. Eisenhower in the 1950s and grew under President John F. Kennedy, who raised the number of American military advisers in South Vietnam from about 700 in 1961 to roughly 16,000 by his death in November 1963. After Kennedy's assassination, President Johnson took office during a period of intensifying communist insurgency in South Vietnam.

The Gulf of Tonkin incident on August 2 and 4, 1964, in which North Vietnamese patrol boats reportedly attacked American destroyers off the Vietnamese coast, gave Johnson the opportunity to seek expanded authority. Congress passed the Gulf of Tonkin Resolution on August 7, 1964 by overwhelming margins of 88 to 2 in the Senate and 416 to 0 in the House, authorizing the president to take all necessary measures to prevent further aggression. After winning the November 1964 election in a landslide, Johnson moved decisively in early 1965.

A Viet Cong attack on the American base at Pleiku on February 7, 1965 killed eight Americans and triggered Operation Flaming Dart retaliatory strikes. On March 2, 1965, Johnson authorized Operation Rolling Thunder, a sustained bombing campaign against North Vietnam that would last until 1968. On March 8, 1965, two battalions of United States Marines, about 3,500 men, came ashore at Da Nang, the first American combat troops deployed to Vietnam. Within weeks, Johnson approved sending Army units as well.

Troop levels rose rapidly. By the end of 1965, about 184,000 American military personnel were in Vietnam. The number reached 385,000 by the end of 1966, 486,000 by the end of 1967, and peaked at roughly 543,000 in April 1969. American combat operations such as the Battle of the Ia Drang Valley in November 1965 produced the first heavy American casualties. Selective Service draft calls climbed sharply, and the Pentagon expanded conscription to fill the ranks. The 1965 escalation transformed an advisory commitment into a major American war.