Vietnam was becoming Communist - The Truman Doctrine meant they should intervene
Kennedy had just succeeded in the Cuban Missile Crisis - overconfidence
USA feared an Asian Domino Theory - China had already fallen, wanted to prevent India falling too, like Vietnam→Laos→Cambodia→Burma→Thailand→India
North Vietnamese patrol boats opened fire on US ships in the Tonkin Gulf in August 1964. This led to the Tonkin Gulf Resolution, which gave Johnson the power to attack Vietnam in a full-scale war.
Containment - stopping the advance of Communism wherever it looked like it was gaining ground
Domestic Politics - being seen to be "tough" on Communism, and continuing Eisenhower's anti-communist policies, was a big vote-winner for Kennedy
Military-Industrial Complex - big business and the military benefited from war. Huge budgets were given to commanders, which were passed on to weapons manufacturers
