US involvement in Vietnam was escalated primarily by Lyndon Johson, and Richard Nixon.
North Vietnamese aggression against Capitalist South Vietnam began in the 1950s as a revolutionary uprising against the colonial government of France. After the French defeat at Diem Bien Phu, in 1955, The US placed advisers and a few Special forces (aka Green Berets) in the Capitalist South Vietnam to aid their resistance to Communist takeover.
After Kennedy was assassinated, Lyndon Johnson became President. He reacted to the rising aggression of the 1960s by upping the Ante, and in 1965 deployed US Army regulars to South Vietnam. The first major engagement in November of 1965, is retold by the book We Were Soldiers Once, and Young by Brig. Gen. (Ret) Harold Moore, and Joe Galloway. The battle was re-enacted in the Movie We Were Soldiers.
As the war dragged on, US casualties mounted, and US desparation resulted in questionable tactics, US Public Opinion turned against the war. This contributed to the election of Richard Nixon as President in 1968. Nixon, however, was determined not to leave Vietnam under a cloud of defeat. He initially escalated US involvement, bombing the Ho Chi Minh trail in Cambodia and Laos, and liberating US troops to launch pre-emptive cross-border raids against Viet-cong (Communist Guerilla fighters) build-ups in Cambodia and Laos.
This successfully forced North Vietnamese leaders to the negotiating table in 1971, and in 1972 a short-lived peace agreement allowed the US to withdraw peacefully. Shortly after the US withdrawal the North resumed hostilities, and conquered South Vietnam in 1974.