 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.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.