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.