The Mayaguez incident, which took place between the Khmer Rouge and the United States from May 12–15, 1975, was the last official battle of the Vietnam War. The names of the Americans killed, as well as those of three U.S. Marines who were left behind on the island of Koh Tang after the battle and who were subsequently executed by the Khmer Rouge, are the last names on the Vietnam Veterans Memorial.
http://military.wikia.com/wiki/Mayaguez_incident