The Vietnam War (1954-1975) was a proxy war, which takes place when opposing powers use third parties to fight on their behalf. During the Vietnam War, the Soviet Union and China backed communist North Vietnam, and the U.S. provided military and economic aid to South Vietnam. In 1963, the South Vietnamese suffered a major defeat to the North Vietnamese Viet Cong in Ap Bac, and the leader of South Vietnam was assassinated. After two American destroyers were sunk, Congress passed the 1964 Gulf of Tonkin Resolution, which authorized President Lyndon B. Johnson to use all necessary means to fight the North Vietnamese. The U.S. chose to send ground troops to Vietnam after Operation Rolling Thunder, a 1965 bombing campaign, did not effectively turn back the North Vietnamese. Marines came ashore at Danang, launching the American ground war in Vietnam. Many Americans turned against the Vietnam War after seeing the horrors of the 1968 Tet Offensive surprise attack aired on nightly TV. On January 27, 1973, a peace agreement was signed, and American forces withdrew from Vietnam in March 1973. When the North Vietnamese invaded the South, Vietnam fell under communist rule.