History of Vietnam since 1945 unstable as no government could keep power for long. There were more coups, often more than one every year. The Communist-run Viet Cong expanded their operation and scored some significant military victories during this period. In 1965, US President Lyndon Johnson sent troops to South Vietnam to secure the country and started to bomb North Vietnam, assuming that if South Vietnam fell to the Communists, other countries in the Southeast Asia would follow, in accordance with the domino theory. Other US allies, such as Australia, New Zealand, South Korea, Thailand, the Philippines, and Taiwan also sent troops to South Vietnam.
1971 in the Vietnam War Exchanges in South Vietnam. President Nixon ordered the initiation of Operation Proud Deep Alpha, an intensive five-day bombing campaign against military targets in North Vietnam just north of they border above the 17th parallel of latitude. The number of U.S. military personnel in South Vietnam totaled 156,800, down from more than 500,000 three years earlier. 1971 in the Vietnam War U.S. military personnel in South Vietnam totaled 334,600 on 31 December 1970. The United States Congress adopted the revised Cooper-Church Amendment which prohibited the introduction of U.S. ground troops or advisers into Cambodia and declared that U.S. aid to Cambodia