The U.S. government viewed American involvement in the war as a way to prevent a Communist takeover of South Vietnam. This was part of a wider containment strategy, with the stated aim of stopping the spread of communism. According to the U.S. domino theory, if one state went Communist, other states in the region would follow, and U.S. policy thus held that Communist rule over all of Vietnam was unacceptable. The North Vietnamese government and the Viet Cong were fighting to reunify Vietnam under communist rule. They viewed the conflict as a colonial war, fought initially against forces from France and then America, as France was backed by the U.S., and later against South Vietnam, which it regarded as a U.S. puppet state.Beginning in 1950, American military advisors arrived in what was then French Indochina. U.S. involvement escalated in the early 1960s, with troop levels tripling in 1961 and again in 1962. U.S. involvement escalated further following the 1964 Gulf of Tonkin incident, in which a U.S. destroyer clashed with North Vietnamese fast attack craft, which was followed by the Gulf of Tonkin Resolution, which gave the U.S. president authorization to increase U.S. military presence. Regular U.S. combat units were deployed beginning in 1965. Operations crossed international borders: bordering areas of Laos and Cambodia were heavily bombed by U.S. forces as American involvement in the war peaked in 1968, the same year that the communist side launched the Tet Offensive. The Tet Offensive failed in its goal of overthrowing the South Vietnamese government but became the turning point in the war, as it persuaded a large segment of the United States population that its government's claims of progress toward winning the war were illusory despite many years of massive U.S. military aid to South Vietnam.Disillusionment with the war by the U.S. led to the gradual withdrawal of U.S. ground forces as part of a policy known as Vietnamization, which aimed to end American involvement in the war while transferring the task of fighting the Communists to the South Vietnamese themselves. Despite the Paris Peace Accord, which was signed by all parties in January 1973, the fighting continued. In the U.S. and the Western world, a large anti-Vietnam War movement developed. This movement was part of a larger Counterculture of the 1960s.