The allied victory in World War II seemed to confirm beyond doubt for Americans that the United States was an exceptional nation with a special role to play in human history. Even before the United States entered the war, Henry Luce, the influential publisher of Life magazine, wrote an essay (17 February 1941) in which he declared that the twentieth century should be considered "the American Century." Luce argued that the United States must "accept wholeheartedly our duty and our opportunity as the most powerful and vital nation in the world and in consequence to exert upon the world the full impact of our influence … [with] … a passionate devotion to great American ideals." By 1945, Luce's vision seemed correct. The United States not only stood victorious but also was the strongest nation in the world militarily and economically. But rather than retreat to the role of exemplar, it remained engaged in world affairs in the postwar years to a greater extent than ever before.
The United States pursued a foreign policy of activist internationalism after World War II as it replaced one global enemy with another. Soviet communism and its containment soon became the focus of U.S. foreign policy as more than forty years of Cold War began. Although strategic, economic and political interests were its central determinants, U.S. Cold War foreign policy and the broad public consensus that supported it were underpinned by an ideology of what John Fousek identifies as "American nationalist globalism" that was rooted in the missionary strand of the belief in American exceptionalism. Anders Stephanson agrees that the "operative framework" of Cold War policy was "the story of American exceptionalism, with its missionary implications."
President Harry S. Truman has been described as "a staunch exponent of American exceptionalism" who frequently referred to the United States as "the greatest nation that the sun ever shone upon." For Truman, the victory in World War II demonstrated American greatness, but it also placed on the United States the responsibility of ensuring peace and freedom in the postwar world. Successive presidents and other public officials and opinion leaders persistently portrayed the Cold War in stark, Manichean terms as a battle between good and evil. The United States was "the leader of the free world" that must prevail and save humanity from the "evils" of communism. Truman provided the guiding principles for American Cold War policy during an address to a joint session of the Congress on 12 March 1947 which became famous for containing the socalled Truman Doctrine. It was the duty of the United States, he contended, to do whatever was necessary to protect the rights of free, democratic nations around the world. In keeping with its traditions, the president claimed, America did not seek dominion over those nations in exchange for their freedom. Truman assured the Congress that "I believe that we must assist free peoples to work out their own destinies in their own way." The task facing the United States was a critical one, he declared, if the values and principles that it held so dear were to survive the challenge of communism and truly enable the world to be led out of darkness: "The free peoples of the world look to us for support in maintaining their freedoms. If we falter in our leadership, we may endanger the peace of the world—and we shall surely endanger the welfare of our own Nation."
Following World War II, President Truman believed that merely to provide an example for the rest of the world to follow would no longer be enough. He was arguing that the United States, as the chosen nation, must take up the gauntlet and defend the rights of free peoples everywhere against what Americans regarded as totalitarian aggression and subversion. The Cold War ethos was, then, firmly grounded in the missionary strand of American exceptionalism. The Truman Doctrine helped define the policy of containment toward the Soviet Union and its allies that was employed by successive U.S. administrations during the cold war. Each of Truman's successors also utilized the language and ideas of American exceptionalism to reinforce the nature of the battle with communism.
Yet it was not only in presidential public rhetoric that the Cold War was defined and discussed in terms of ideas about American destiny, duty and exceptionalism. For example, George Kennan, the original architect of the containment policy, ended his influential article "The Sources of Soviet Conduct" in the journal Foreign Affairs (July 1947) by arguing: "The issue of Soviet-American relations is in essence a test of the overall worth of the United States as a nation among nations. To avoid destruction the United States need only measure up to its own best traditions and prove itself worthy of preservation as a great nation." He claimed that the "thoughtful observer" would not object to the Cold War but would "rather experience a certain gratitude to a Providence which, by providing the American people with this implacable challenge, has made their entire security as a nation dependent on their pulling themselves together and accepting the responsibilities of moral and political leadership that history plainly intended them to bear." Kennan, who was often critical of moralism in foreign policy, had nonetheless used the traditional language of exceptionalism to advocate his strategy for containing Soviet communism.
During the Cold War, as Michael Hunt has observed, the private communications of policymakers and even secret national security documents were frequently "couched in the stark and sweeping terms usually reserved for crusades." For example, the authors of the secret National Security Council Paper Number 68 (NSC 68), the document that defined the course of U.S. Cold War policy in 1950, made clear that "Our position as the center of power in the free world places a heavy responsibility upon the United States for leadership." They described the Cold War as "a basic conflict between the idea of freedom under a government of laws, and the idea of slavery under the grim oligarchy of the Kremlin." It was "imperative" that the forces of "freedom" prevail, so the United States must, therefore, build up its political, economic, and military strength. This document was designed only for the eyes of other policymakers yet it was built around the idea of free world leadership that, as Fousek argues, "became the controlling metaphor in U.S. foreign policy discourse throughout the postwar period."
Many Cold War policies reflected exceptionalist assumptions about the American role in the world. The Marshall Plan (1947) for the economic reconstruction of postwar Western Europe was designed to revive European economies using not only American money but also practices and principles. The United States, in the tradition of the redeemer nation, was fulfilling its responsibility of leadership through a program that not only provided benefits for itself but also for the peoples of war-torn Europe. Similarly, modernization theory provided the rationale for much U.S. Cold War policy toward the developing world, particularly during the Kennedy administration. Modernization theorists believed that all societies pass through sequential stages of progress from "traditional" to "modern" and that the West, and in particular the United States, was the "common endpoint" to which all peoples must irresistibly move. Significantly, these influential social scientists also argued that the United States could help underdeveloped countries along the way from being stagnant, traditional societies to active, modern ones—a transition which would not only bring advantages to the peoples of those societies but also to the United States. What made this theory so appealing, according to Michael Latham, was that it gave an "objective" justification for U.S. intervention in other nation's affairs; it provided a useful weapon in the Cold War struggle with the Soviet Union; and it also fit with American values and traditions, in particular the notion that the United States is an exceptional nation with a mission and duty to be the engine of human progress. As Latham observes "modernization reinforced the connections between strategic concerns and assumptions about America's role as a moral, benevolent world leader." Modernization theory shaped the formation, influenced the major practices, and informed the public presentations of so-called "nation building" programs such as the Alliance for Progress with Latin America, the Peace Corps, and the Strategic Hamlet Program in Vietnam. Each program was designed to exemplify the altruistic, benevolent impulses of the United States while also being advocated as an objective, scientifically proven method for aiding developing nations. In the early 1960s, Latham contends, the strength and influence of modernization as an ideology ensured the "continuing power of the widespread belief that America was both called to and capable of remaking the rest of the world."
Not all Americans, however, accepted uncritically the tenets of the Cold War consensus. African Americans in particular focused on the apparent contradiction of the U.S. demanding freedom and democracy throughout the world when such rights were still widely denied to large numbers of its own citizens at home. The civil rights movement used claims of America's leadership of the free world to argue that racial equality must also be achieved in the United States. Such demands contributed to the many advancements made in race relations during the 1950s and 1960s. On the whole, though, raising objections to the Cold War consensus was difficult, even dangerous, particularly during the earlier years of the Cold War, not least because foreign policy dissent was frequently equated with "fundamental disloyalty to the nation and its values."
By the late 1960s, however, much of the Cold War consensus had begun to break down and large numbers of Americans were questioning the direction of U.S. foreign policy, particularly toward Vietnam. The intervention in Vietnam had been conceived in terms consistent with the belief that the United States was the leader of the free world with a duty to contain communism and protect "free peoples" from aggression. But the war's conduct and the resultant defeat of American objectives would raise serious doubts about both the Cold War consensus and the exceptional nature of the United States.