America's limited war theory, which provided the intellectual justification and guide for the U.S. involvement in Vietnam, suffered from all the flaws which Clausewitz had seen in such abstract approaches to war 130 years earlier. Misled by the apparent rationalist perfection of its theory, U.S. leaders failed to understand that a war of limited objectives and means is only possible when both sides are willing to restrict means. They could not understand this because their theory did not admit the role of passion and will in driving a people's effort in war. This paper will discuss the series of errors in strategic thinking that flowed from this fundamental misunderstanding of the nature of war, and which led to the United States' loss of the Vietnam War. First, the U.S. did not recognize that the true Clausewitzian "center of gravity" in Vietnam was essentially political: the will of the people to resist the Communist insurgency, a will the United States could not create or substitute for by military action. The United States thus chose a military objective - defeat of the North Vietnamese Army - when it became clear that Saigon was losing its grip on the country in 1965. Second, having engaged the North Vietnamese, U.S. leadership did not understand the role of moral factors in determining the amount of forces which North Vietnam and the Viet Cong would field and which the United States would have to match. Totally committed to victory in the South, the North Vietnamese continued to up the ante. Third, the United States, seeking a military solution to a political problem, used the military as a political tool rather than directing it to a clear military objective, thereby increasing the cost of the war. Having lost control of the cost of the war, the United States leadership lost the war because it lost the support of the American people.