Realists argue that states pursue national interest, which they define primarily as power and security. The national interest is not fixed but is shaped by the state's position in the international system and the threats it faces. Understanding what states want—power, security, and relative advantage—is central to explaining their behavior.
Examine case studies where leaders explicitly articulated their country's national interest (e.g., Cold War containment doctrine, imperial powers' rationales for expansion) and compare with stated idealistic goals.
National interest is not synonymous with GDP or economic growth; a state can sacrifice economic gains for security. Nor is it immoral or amoral—states often pursue values alongside interests.
Your prerequisite study of classical realism and Morgenthau gave you the foundational realist claim: international politics is governed by the pursuit of power, and understanding state behavior requires understanding power. National interest is the concept that operationalizes this claim — it is the set of objectives that a state pursues in international affairs, understood primarily through the lens of security, survival, and relative position in the international hierarchy.
Morgenthau's formulation is the anchor: the national interest defined in terms of power. This deliberately strips away ideological justifications and moral rhetoric to expose what states actually do. When the United States contains Soviet expansion, when Britain maintains the European balance of power, when small states bandwagon with great powers — in each case, realists argue, the underlying driver is interest defined by the state's position and vulnerability in the international system. This is not cynicism but a methodological claim: to predict and explain state behavior, focus on structural incentives and capability distributions, not on the stated intentions of leaders.
The distinction between power and security as the primary national interest is more important than it may initially appear. Offensive realists like John Mearsheimer argue that states are never satisfied with their current power position — they seek hegemony because relative advantage is the only reliable guarantee of security in an anarchic system. Defensive realists argue that most states are security-seekers rather than power-maximizers: they want enough power to be secure, but aggressive expansion beyond that point is typically counterproductive because it triggers balancing coalitions. The debate matters because it generates different predictions about the circumstances under which states will start wars, when great powers will be satisfied, and whether international cooperation is stable.
A key insight is that the national interest is not simply discovered — it is constructed through domestic political processes. Leaders interpret international threats and opportunities through ideological frameworks, historical analogies, and bureaucratic pressures. Two states in identical structural positions might define their national interest differently based on elite ideology or domestic coalition pressures. The 1930s United States and Germany faced similar post-Depression domestic conditions but pursued radically different foreign policies. This complicates the realist framework: if national interest were simply read off from the state's structural position, we would expect more convergence in behavior than we actually observe. The concept remains analytically useful, but explaining variance in how states define their interests requires supplementing structural realism with attention to domestic politics and ideology — which is where IR theory has increasingly moved.
Relative gains versus absolute gains captures a classic realist tension. If two states are considering a trade agreement that benefits both, realists note that states may refuse cooperation that offers greater relative gains to their partner, even if both gain in absolute terms, because relative position determines future military and economic capacity. A state that gains 10% from a deal while its rival gains 20% has become relatively weaker. This explains why purely interest-based calculations can impede cooperation even when cooperation is mutually beneficial — and why international institutions that make relative gains transparent or reduce their significance can facilitate cooperation that pure power politics would otherwise block.
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