Hans J. Morgenthau’s Politics Among Nations (1948) helped to meet the need for a general theoretical framework. Not only did it become one of the most extensively used textbooks in the United States and Britain—it continued to be republished over the next half century—it also was an essential exposition of the realist theory of international relations. Numerous other contributors to realist theory emerged in the decade or so after World War II, including Arnold Wolfers, George F. Kennan, Robert Strausz-Hupé, Kissinger, and the theologian Reinhold Niebuhr.

Although there are many variations of realism, all of them make use of the core concepts of national interest and the struggle for power. According to realism, states exist within an anarchic international system in which they are ultimately dependent on their own capabilities, or power, to further their national interests. The most important national interest is the survival of the state, including its people, political system, and territorial integrity. Other major interests for realists include preservation of the culture and the economy. Realists contend that, as long as the world is divided into nation-states in an anarchic setting, national interest will remain the essence of international politics. The struggle for power is part of human nature and takes essentially two forms: collaboration and competition. Collaboration occurs when parties find that their interests coincide (e.g., when they form alliances or coalitions designed to maximize their collective power, usually against an adversary). Rivalry, competition, and conflict result from the clash of national interests that is characteristic of the anarchic system. Accommodation between states is possible through skillful political leadership, which includes the prioritizing of national goals in order to limit conflicts with other states.

In an international system composed of sovereign states, the survival of both the states and the system depends on the intelligent pursuit of national interests and the accurate calculation of national power. Realists caution that messianic religious and ideological crusades can obscure core national interests and threaten the survival of individual states and the international system itself. Such crusades included, for Morgenthau, the pursuit of global communism or global democracy, each of which would inevitably clash with the other or with other competing ideologies. The attempt to reform countries toward the ideal of universal trust and cooperation, according to realists, runs counter to human nature, which is inclined toward competition, conflict, and war.

Realist theory emerged in the decade after World War II as a response to idealism, which generally held that policy makers should refrain from immoral or illegal actions in world affairs. As no impressive new formulation of political idealism appeared on the international scene to reply to realist theory, the debate between realism and idealism gradually faded, only to be revived in a somewhat different form in the final decades of the 20th century in the disagreement between neoliberal institutionalists and neorealist structuralists.

Many international relations scholars neither rejected nor embraced realism but instead were engrossed in other aspects of the broadening agenda of international relations studies. Beginning in the 1950s, as the United States became more fully engaged in world affairs, the U.S. government made large sums of money available for the development of area studies, especially studies of regions that were important in the intensifying Cold War with the Soviet Union. In order to understand the major forces and trends shaping countries such as the Soviet Union and China or the regions extending from Africa to Northeast Asia, the United States needed to recruit greater numbers of specialists in the histories, politics, cultures, economies, languages, and literature of such areas; the Soviet Union did likewise. Theoretical concerns generally played a marginal role in the growth of area specialization in the West. Although many scholars agreed with Morgenthau’s statement that theory and research should have a “concern with human nature as it actually is, and with the historic processes as they actually take place,” they did not uniformly believe that realism was capable of providing an adequate explanation of international behaviour.

The behavioral approach and the task of integration

In the 1950s an important development in the social sciences, including the study of international relations, was the arrival of new concepts and methodologies that were loosely identified in ensemble as behavioral theory. This general approach, which emphasized narrowly focused quantitative studies designed to obtain precise results, created a wide-ranging controversy between theorists who believed that the social sciences should emulate as much as possible the methodologies of the physical sciences and those who held that such an approach is fundamentally unsound. In addition, the great number of new topics investigated at the time—including cognition, conflict resolution, decision making, deterrence, development, the environment, game theory, economic and political integration, and systems analysis—provoked some anxiety that the discipline would collapse into complete conceptual and methodological chaos. Accordingly, much of the intellectual effort of the mid-1950s to mid-1960s—the so-called “behavioral decade”—went into the task of comparing, interpreting, and integrating various concepts from new areas of study, and the scholarly goal of the period was to link theories, or to connect so-called “islands of theory,” into a greater, more comprehensive theory of international relations.

This task proved to be a difficult one. Indeed, some scholars began to question the necessity—or even the possibility—of arriving at a single theory that would explain all the varied, diverse, and complex facets of international relations. Instead, these researchers suggested that a number of separate theories would be needed.

At the same time, theories that trace the forces of international relations to a single source were increasingly viewed as unsatisfactory. The struggle for power, for example, was accepted as a fact in past and current international politics, but attempts to make all other factors subordinate to or dependent upon power were thought to exclude too much of what is important and interesting in international relations. Similar assessments were made of the theory that asserts that the character of a nation—and hence the character of its participation in international relations—is determined by its child-rearing practices, as well as of the Marxist theory that international relations are solely the historical expression of class struggle.

The general attitude of the behavioral decade was that the facts of international relations are multidimensional and therefore have multiple causes. This conclusion supported, and in turn was supported by, the related view that an adequate account of these facts could not be provided in a single integrated theory and that multiple separate theories were required instead. By the 1960s, for example, studies of international conflict had come to encompass a number of different perspectives, including the realist theory of the struggle for power between states and the Marxist notion of global class conflict, as well as other explanations. At the same time, conflict theory coexisted with economic and political integration theory and game theory, each of which approached the phenomena of international conflict from a distinct perspective.

In keeping with the multiple-theory approach, by the end of the behavioral decade there was a growing consensus that the study of international relations should encompass both quantitative and qualitative analyses. Whereas quantitative methodologies were recognized as useful for measuring and comparing international phenomena and identifying common features and patterns of behaviour, qualitative analyses, by focusing on one case or a comparison of cases involving specific research questions, hypotheses, or categories, were thought to provide a deeper understanding of what is unique about political leaders, nations, and important international events such as World War II and the Cold War.

The use of quantitative analysis in international relations studies increased significantly in the decades after the 1960s. This was a direct result of advances in computer technology, both in the collection and retrieval of information and in the analysis of data. When computers were introduced in international relations studies, it was not readily apparent how best to exploit the new technology, partly because most earlier studies of international relations were set forth in narrative or literary form and partly because many of the phenomena examined were not easily quantifiable. Nevertheless, exploratory quantitative studies were undertaken in a number of directions. A growing body of studies, for example, developed correlations between phenomena such as alliances and the outbreak or deterrence of war, between levels of political integration and levels of trade, communication, and mobility of populations, between levels of economic development and internal political stability, and between levels of internal violence and participation in international conflicts.