The McNamara Line, an operational strategy employed by the United States in 1966–1968 during the Vietnam War, aimed to prevent infiltration of South Vietnam by NVA forces from North Vietnam and Laos. Physically, the McNamara Line ran across South Vietnam from Cửa Việt port to Route 9 and to the Laotian border along the Vietnamese Demilitarized Zone (DMZ) till Mường Phìn, Laos. The eastern part included fortified field segments, with Khe Sanh as linchpin, along with stretches where roads and trails were guarded by high-tech acoustic and heat-detecting sensors on the ground and interdicted from the air.^: 349 Assorted types of mines, including so-called gravel mines, and troops at choke points backed sophisticated electronic surveillance. Named the barrier system by Robert McNamara (United States Secretary of Defense from 1961 to 1968), it was one of the key elements, along with gradual aerial bombing, of his war strategy in Vietnam.^{: 508–509}