These two modes of perceiving world politics were never uniquely American in precept or experience. Western political thought always recognized the tension between realist and idealist views toward the actions of governments in both domestic and international transactions. The stark realism of Niccolò Machiavelli stood in profound opposition to the dominant Christian teachings that favored ethical constraints upon rulers. In the eighteenth century, doctrines of raison d'état contended with Enlightenment doctrines propounded by philosophers who objected to such practices of monarchical statecraft as mercantilism, balance-of-power politics, and the pursuit of dynastic goals at the expense of peace and human welfare.
While the American clash between realism and idealism owes an intellectual debt to antecedent European thought, it was in the United States that both doctrines were fully established, in theory and in practice. Whereas in continental Europe, utopian idealism remained excluded from the realm of practice, in the United States it became a recurrent, contrapuntal theme of statesmen and politicians, commentators and theorists. What underlay the conflicting presumptions regarding the requirements and possibilities of external action was the anarchical nature of the international environment. Whereas governmental structures within established countries assured some degree of order and security, the absence of international authority compelled individual countries to fend for themselves, relying on their own capacities to coexist in what social contract theorists termed a state of nature. Realists and idealists disagreed totally over the capacity of human society, and especially international politics, to eliminate the vagaries of existence in an anarchic state system.
Realists, recognizing no genuine alternative to coexistence in an anarchical world of individual sovereign nations, accepted the modern state system as a necessity. They would defend the country's interests by following the rules of diplomacy and war as propounded by a host of seventeenth-and eighteenth-century writers and statesmen. These rules of conduct were not designed to prevent conflict and war, but rather to mitigate their effects and thereby assure the survival of states. For realists, moreover, war was not an aberration, but a condition sometimes unavoidable, a contingency for which to prepare, but also, when possible, to deter by force or accommodation. Wars, they knew, were generally the only means available for changing unwanted political or territorial conditions. Realists thus accepted power politics as a natural phenomenon of international life, with the concomitant reliance on armies and navies, secret diplomacy, and alliances. Asserting the primacy of national over individual interests, they viewed the universal norms governing human rights as conditional when they threatened the national welfare. Realists observed the essential truth that nations existed successfully amid the world's anarchy. The evidence lay in the precedence of peace over war, as well as the continued material advancement in human affairs.
Idealists viewed the international system, with its accoutrements of conflict and war, as not only deeply flawed but also capable of melioration, if not total cure. For them, international strife was the unnecessary and reprehensible product of outmoded forms of human organization, both in the internal structuring of states and in their international practices. Idealists saw in the trappings of power politics little but ambition, opportunism, deception, and impositions. Whereas realist doctrine focused on national interests and security, idealist concerns looked to individual welfare and the general interests of humanity. Idealists presumed that the objective validity and authority of universal norms, laws, and principles could and should apply to international as well as domestic affairs.
Realists and idealists disagreed fundamentally on the primary determinants of state behavior in international politics. For realists, external factors defined the options available to policy-makers. Those options were uncertain and elusive, requiring preparedness as well as caution. Secretary of State Dean Acheson once remarked: "The future is unpredictable. Only one thing—the unexpected—can be reasonably anticipated…. The part of wisdom is to be prepared for what may happen, rather than to base our course upon faith in what should happen." The German historian Leopold von Ranke formulated this view in terms congenial to American realists. The dangers and uncertainties of international life, he wrote, not only established the primacy of foreign affairs but also dictated the precedence of security interests over domestic concerns. While cognizant of the historical vicissitudes in national fortunes, realists nevertheless saw constancy in the essential traits and behavior of nations. Policies might vary with regimes, but fundamental interests, once established, tended to remain consistent.
Idealists, on the contrary, tended to view the sources of external state action as residing in internal political processes, based largely on political structures, the distribution of political power, and the ambitions of ruling elites. Involvements abroad reflected not external necessity, but internal choice. To idealists, different forms of government led to different modes of foreign policy. Autocratic states, some idealists presumed, too readily threatened the cause of humanity by placing demands on individuals that were sharply at odds with private conscience. By ordering men into mortal combat with other members of the human race, they shattered the peace and defied the civilized norms of human conduct. Authentic republics did not wage aggressive wars, nor did free peoples impose imperial control over others.
However apparent the wellsprings of aggressive national behavior, realists accepted limits on both their intentions and their power to interfere. They recognized the barriers that national sovereignty placed on meliorist efforts to alter the political structures and domestic decisions of other countries. Idealists, as children of the Enlightenment, expected more of themselves and society. For them, the world was not hopelessly corrupt, but could, through proper leadership and motivation, advance morally and politically. This optimistic view of the world became endemic to the idealists' presumptions of human progress and the concomitant conviction that the United States, because of the superiority of its institutions, was ideally constituted to lead the world toward an improving future. The belief that institutional and moral superiority distinguished the United States from other countries found its central expression in the concept of "exceptionalism." This assigned to American suppositions of exceptional virtue the imperative of exceptional obligation to serve the peace and improve the human condition.