"One reason for the failure of Johnson's Vietnam policy was the inherent unworkability of U.S. military strategy. The gradual escalation of the U.S. bombing campaign allowed the North Vietnamese sufficient time to disperse their population and resources and to develop an air defense system that would destroy a large number of U.S. aircraft. Moreover, the U.S. Army never developed a consistent strategy for stopping the infiltrations of regular North Vietnamese units and supplies into the South. General Westmoreland's search-and-destroy strategy was designed primarily to protect the cities of South Vietnam while killing as many Vietcong as possible. Westmoreland grossly miscalculated North Vietnam's willingness to suffer huge losses in manpower as well as its capacity to replace those losses. An estimated 200,000 North Vietnamese males reached draft age each year, far more than U.S. forces could kill. North Vietnam was able to sustain its war effort by drawing on both Soviet and Chinese military and economic assistance. With the Sino-Soviet split deeper than ever, even after Krushchev's demise, both communist powers tried to outdo each other in helping North Vietnam. Their combined assistance between 1965 and 1968 exceeded $2 billion, an amount that more than offset the losses North Vietnam suffered from U.S. bombing. In addition, between 1962 and 1968 approximately 300,000 Chinese soldiers went to North Vietnam, 4,000 of whom were killed. Though not participating in ground combat, they helped operate antiaircraft weapons and communications facilities."