After its victory in World War II, it was clear that United States should move beyond the disastrous policies of the 1930s, but it was less clear how. Ultimately, a lasting postwar strategy was forged under President Truman. Appreciating how Truman moved well beyond Roosevelt’s guiding assumptions is essential to understanding the evolution of American grand strategy. One sees that wartime planning and grand strategy formulation can prove quite inadequate for dealing with postwar challenges. An administration cannot be locked into assumptions, but must constantly test them. Thus, the Truman administration eventually developed and adopted containment and moved far beyond FDR’s approach. More substantively, the fundamental geopolitical lesson of World War II and the early Cold War was that the United States must assume the essential balancing role relative to other major powers.