After President Lyndon Johnson reports that North Vietnamese patrol boats have fired on American naval vessels in the Gulf of Tonkin, Congress passes the Gulf of Tonkin Resolution. It authorizes the President to take all necessary measures to repel another armed attack and to prevent further aggression. President Johnson later uses the Gulf of Tonkin Resolution as a declaration of war enabling him to commit several hundred thousand American troops to South Vietnam. The United States withdraws its troops from South Vietnam in 1973, after signing a peace treaty. Hostilities between the North and South continue until Congress finally cuts off all military aid to the South in 1975. North Vietnam prevails and unites Vietnam under its rule.