The Vietnam War was an undeclared war in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. It was the second of the Indochina Wars and was officially fought between North Vietnam and South Vietnam. North Vietnam was supported by the Soviet Union, China, and other communist allies; South Vietnam was supported by the United States, South Korea, the Philippines, Australia, Thailand and other anti-communist allies. It lasted some 19 years, with direct US involvement ending in 1973 following the Paris Peace Accords, and included the Laotian Civil War and the Cambodian Civil War. Vietnam, Laos, and Cambodia all became communist states in 1975 as a result of the war. In March 1959, Ho Chi Minh declared all out war to unite Vietnam under one rule. In December 1961, US military advisors began to take a direct role in the war. In August 1964, after two US destroyers were attacked by the North Vietnamese, the Gulf of Tonkin Resolution was passed by the US Congress, allowing US troops to use armed force in the area. On March 8, 1965, the first official US combat troops arrived in Vietnam, and the US began a bombing campaign of Northern Vietnam called Operation Rolling Thunder. On January 30, 1968, North Vietnam launched the Tet Offensive, attacking around 100 cities in Southern Vietnam. In July 1969, President Nixon began the withdrawal of US troops. In March 1972, the North Vietnamese attacked across the border in the Easter Offensive. On January 27, 1973, a ceasefire agreement was reached between US and North Vietnam and US POWS began to return home. On March 29, 1973, the last US combat troops left South Vietnam.