 From spring through the fall of 1967 , the U.S. Command in Saigon was perplexed by a series of actions initiated by the North Vietnamese and the Viet Cong in the border regions . On 24 April a U.S. Marine Corps patrol prematurely triggered a North Vietnamese offensive aimed at taking the airstrip and combat base at Khe Sanh , the western anchor of the Marines ' defensive positions in Quảng Trị Province . By the time the action there had ended in May , 940 North Vietnamese troops and 155 Marines had been killed . For 49 days during early September and lasting into October , the North Vietnamese began shelling the U.S. Marine outpost of Con Thien , just south of the Demilitarized Zone or DMZ . The intense shelling ( 100 – 150 rounds per day ) prompted Westmoreland to launch Operation Neutralize , an intense aerial bombardment campaign of 4 @,@ 000 sorties into and just north of the demarcation line . 
