High Quality Content by WIKIPEDIA articles! In the Vietnam War, after the assassinations of Ngo Dinh Diem and John F. Kennedy in late 1963 and the Gulf of Tonkin Incident in 1964 and the continuing political instability in the South, the United States made a policy commitment to begin joint warfare in South Vietnam, a period of gradual escalation and Americanization, involving the commitment of large-scale combat forces from the United States and allied countries. It was no longer assumed the Republic of Vietnam could create a desirable situation without major external assistance. This phase of the war lasted until the election of Richard Nixon, and the change of U.S. policy to Vietnamization, or giving the main combat role back to the South Vietnamese military.