1. The Domino Theory: The Domino Theory was the idea that if one Southeast Asian country fell to communism, the others would soon follow. This theory led the United States to support South Vietnam in the hope of preventing the spread of communism in Southeast Asia.
2. The Gulf of Tonkin Resolution: The Gulf of Tonkin Resolution was passed by Congress in 1964 after North Vietnamese forces allegedly attacked two American destroyers in the Gulf of Tonkin. The resolution gave President Lyndon B. Johnson broad authority to wage war in Vietnam.
3. The Tet Offensive: The Tet Offensive was a massive North Vietnamese offensive that began in January 1968. The offensive was a turning point in the war, as it showed that the United States and South Vietnam could not win the war militarily.