 By 1966 – 1967 , however , after suffering massive casualties , stalemate on the battlefield , and destruction of the northern economy by U.S. aerial bombing , there was a dawning realization that , if current trends continued , Hanoi would eventually lack the resources necessary to affect the military situation in the South . As a result , there were more strident calls by the moderates for negotiations and a revision of strategy . They felt that a return to guerrilla tactics was more appropriate since the U.S. could not be defeated conventionally . They also complained that the policy of rejecting negotiations was in error . The Americans could only be worn down in a war of wills during a period of " fighting while talking . " During 1967 things had become so bad on the battlefield that Lê Duẩn ordered Thanh to incorporate aspects of protracted guerrilla warfare into his strategy . 
