The regimental headquarters arrived in South Vietnam on 26 April 1967. In mid-May 1967, following the conclusion of The Hill Fights, the 26th Marines took over responsibility for the area around Khe Sanh from the 3rd Marines as Operation Crockett. The 26th Marines conducted Operation Ardmore a search and destroy mission in the Khe Sanh area from 17 July-31 October 1967. At the conclusion of Operation Ardmore the Marines commenced Operation Scotland, the defense of Khe Sanh Combat Base and search and destroy missions against People's Army of Vietnam (PAVN) infiltration. From 21 January 1968 the 26th Marines were under siege at Khe Sanh until the conclusion of Operation Pegasus on 14 April 1968 and were replaced by the 1st Marines on 15 April 1968. The 26th Marines were awarded a Presidential Unit Citation for their actions during the Battle of Khe Sanh as were the Seabees that supported them.