Classical realist theories of international relations posit the national interest as the fundamental, irreducible motivation behind state actions. This concept, though seemingly straightforward, encompasses a complex interplay of security, power, and survival that states perpetually pursue in an anarchically structured international system. From the ancient observations of Thucydides to the mid-20th-century formulations of Hans Morgenthau, a consistent thread emerges: states act to maximize their advantage and ensure their continued existence, and this pursuit is primarily defined by what best serves their national interest. Understanding this concept is crucial to grasping the realist worldview, which often views international politics as a perpetual struggle for power driven by this core principle.
Thucydides, in his account of the Peloponnesian War, offers an early and influential articulation of the national interest in action. His famous dialogue between the Athenian envoys and the Melians in 416 BCE starkly illustrates the realist perspective. The Athenians, facing a neutral Melos, declared, "the strong do what they can and the weak suffer what they must." This statement, devoid of moral or legalistic pretense, reflects a pragmatic assessment of power dynamics and self-preservation. For Athens, the national interest lay in expanding its empire and crushing potential threats, even if it meant subjugating a weaker state. Melos, in contrast, appealed to divine justice and the potential support of Sparta, but its understanding of its own interest—survival through neutrality and hope—proved tragically misjudged against the overwhelming power and perceived necessity of Athenian expansion. Thucydides shows how states, when confronted with existential or significant strategic choices, invariably prioritize what they deem necessary for their own security and dominance.
Centuries later, Hans Morgenthau, perhaps the most prominent figure in 20th-century classical realism, codified these ideas in Politics Among Nations. Morgenthau’s first principle of realism is that "politics, like society in general, is governed by objective laws that have their roots in human nature." Human nature, for Morgenthau, is inherently self-interested and power-seeking, which in turn shapes the national interest. He defines the national interest in terms of power, arguing that states perpetually strive to increase their power or maintain the existing balance of power to ensure their security and autonomy. This pursuit is not necessarily born of malice but of a fundamental drive for survival in a dangerous world. For instance, Morgenthau would analyze the Cold War arms race not as an ideological battle, but as a direct manifestation of the United States and the Soviet Union acting to secure their national interests through military buildup, each perceiving the other as a threat to their very existence and global standing. The national interest, in this view, becomes synonymous with the accumulation and deployment of power to safeguard the state’s sovereignty and influence.
The concept of the national interest also accounts for the apparent cyclical nature of international relations. Realists argue that because the international system is anarchic—lacking a supreme authority to enforce rules—states must constantly be vigilant. This perpetual insecurity compels them to act in ways that enhance their capabilities relative to others. A state might pursue alliances not out of affection, but because they serve its interest in collective security or power projection. Similarly, a state might engage in economic policies that benefit domestic industries, even at the expense of international cooperation, if it perceives that economic strength directly translates into national power and security. The shifting alliances and rivalries observed throughout history, from the Concert of Europe in the 19th century to the post-World War II bipolar world, can be understood through the lens of states re-evaluating and adjusting their pursuit of the national interest in response to changing power distributions and threats.
In conclusion, classical realist theories consistently foreground the national interest as the primary determinant of state behavior. This interest is not a static or morally defined objective, but a dynamic and often power-centric pursuit of security, survival, and influence within an anarchic global arena. Whether articulated by ancient historians or modern political scientists, the core tenet remains: states act to secure themselves and advance their positions, and this imperative is encapsulated by the concept of the national interest. This perspective offers a powerful, albeit often stark, framework for understanding the enduring patterns of conflict and cooperation in international affairs.