The United States becomes involved in the Vietnam War to stop the spread of communism in Southeast Asia. Over time, the war becomes increasingly unpopular and divisive in the US as it drags on without clear progress. Protests and opposition to the war intensify during the 1960s. Finally, in the early 1970s, President Nixon begins withdrawing US troops from Vietnam as part of his Vietnamization policy. The last US troops depart in 1973 after over a decade of involvement, leaving South Vietnam to fight on its own.