The Foundations of Classical Realism in International Relations

Why do states arm themselves even when no one has attacked them? Why does diplomacy so often fail to prevent war? Classical realism, one of the oldest and most influential theories in International Relations, offers a blunt answer: the world is a perpetual struggle for power, and that struggle is rooted in human nature itself. This worldview, built over more than two thousand years by thinkers from ancient Athens to twentieth-century America, still shapes how diplomats, strategists, and scholars read global politics today. Understanding it is the starting point for making sense of every other theory that came after.

What is classical realism? 🔗

Classical realism is a theory of international relations that explains state behaviour through human nature, the pursuit of power, and the anarchic structure of the world. At its core, it makes a few firm assumptions: states are the main actors in global politics, there is no supranational authority above them, states act in their own self-interest, and they seek power above all to ensure their own survival. What sets classical realism apart from later versions of realism is where it locates the source of this behaviour. It points directly to human nature and the moral character of individuals and leaders rather than to the structure of the system alone.

The theory rose to prominence in the middle of the twentieth century, largely as a reaction against idealism, the optimistic belief that international cooperation, law, and institutions could permanently tame conflict. After the failure of the League of Nations and the catastrophe of the Second World War, realists argued that such optimism ignored the harsh realities of power. They wanted a theory grounded in history and human behaviour as they actually are, not as we might wish them to be.

The pessimistic view of human nature 🔗

Everything in classical realism flows from one foundational idea: human beings are inherently driven by self-interest, insecurity, and a desire for power. This is not an incidental feature of human life but its defining trait. Greed, fear, and the will to dominate are seen as permanent parts of the human condition, present in every era and every culture.

Because states are run by people, this same nature carries upward to the level of the state. A nation behaves much like an individual writ large: it competes, fears its rivals, and seeks to expand its influence. This is why classical realists are often called pessimists. They do not believe that education, prosperity, or moral progress can erase the underlying drive for power. The English philosopher Thomas Hobbes gave this idea its sharpest expression. He described a “state of nature” without any common authority as a condition of constant insecurity, and realists apply this image directly to the international arena, where no world government exists to keep order.

The thinkers who built the theory 🔗

Classical realism is not the work of a single author. It is a tradition stitched together across centuries, drawing on historians, philosophers, and political theorists who each described the same essential truth about power.

Thucydides and the Peloponnesian War 🔗

The intellectual roots of classical realism reach back to ancient Greece. Thucydides, an Athenian historian, is often considered the earliest realist thinker. In his account of the Peloponnesian War between Athens and Sparta, he showed how the struggle for power, not justice or morality, drove the behaviour of city-states.

The most famous illustration of his thinking is the Melian Dialogue, where the powerful Athenians tell the weaker Melians that the strong do what they have the power to do while the weak accept what they must. This single episode captures the realist conviction that in international politics, power often determines outcomes far more than principle. Thucydides also identified fear, honour, and interest as the deep motives behind war, a framework that realists still use to analyse conflict today.

Machiavelli and the logic of necessity 🔗

The Italian Renaissance thinker Niccolò Machiavelli added a crucial dimension in his work The Prince. He argued that a ruler must be guided by pragmatism rather than conventional morality. For Machiavelli, the survival and security of the state justified actions that would be considered immoral for a private individual. A leader had to be willing to act with cunning and, when necessary, ruthlessness, because the world rewards effectiveness, not good intentions.

This is the origin of an idea central to classical realism: in international politics, principles are subordinate to policies. Survival comes first, and the means used to secure it are judged by their results. Machiavelli’s “doctrine of necessity” holds that states will inevitably be drawn into conflict, which forces leaders to prioritise self-preservation even at the cost of morality.

Hans Morgenthau, the father of classical realism 🔗

If Thucydides and Machiavelli laid the foundations, it was Hans Morgenthau, a German-American scholar, who turned these scattered insights into a systematic theory. Having fled Nazi Germany, Morgenthau understood firsthand how unchecked power could lead to catastrophe. In 1948 he published Politics Among Nations: The Struggle for Power and Peace, which became one of the most influential textbooks in the field and is widely regarded as the defining statement of classical realism.

Morgenthau’s six principles of political realism 🔗

Morgenthau distilled his theory into six principles of political realism, which he added as an introductory chapter to later editions of his book. These principles remain a standard reference point for students of international politics.

Objective laws rooted in human nature: Politics, like society in general, is governed by objective laws that have their roots in an unchanging human nature. To understand international politics, we must first grasp these laws and build a rational theory around them.

Interest defined as power: This is the master key of the entire theory. Morgenthau argued that the concept of interest defined in terms of power is the signpost that helps us navigate the landscape of international politics. Statesmen think and act in terms of power, which allows us to analyse their behaviour rationally.

Interest is dynamic, not fixed: While interest is always defined as power, the actual content of national interest changes with the political and cultural environment. What power means in one era or region may differ in another, so the idea must be applied to the concrete circumstances of time and place.

Tension between morality and political action: Realism is aware of the moral significance of politics, but it insists that universal moral principles cannot be applied to the actions of states in their abstract form. Morality must be filtered through the practical realities a leader faces.

No nation’s morality is universal: Realism refuses to identify the moral aspirations of any single nation with the moral laws that govern the universe. Believing that your own country’s interests are sacred and universal leads, in Morgenthau’s view, to moral excess and political folly.

The autonomy of the political sphere: Just as an economist asks how a policy affects wealth and a lawyer asks whether it conforms to legal rules, the political realist asks how a policy affects power. The political dimension stands on its own and cannot be reduced to economics, law, or ethics.

International politics as an endless struggle for power 🔗

Putting these ideas together, classical realism presents a clear picture of world politics. Because there is no overarching authority to enforce order, the international system is anarchic. Every state is ultimately responsible for its own security. This condition produces a relentless competition where each nation tries to protect and expand its power relative to others.

One consequence of this is the security dilemma. When one state builds up its military to feel safer, its neighbours interpret this as a threat and respond by arming themselves, leaving everyone less secure than before. Actions taken purely for defence can unintentionally raise tensions and trigger arms races. War, in this view, is not an aberration but a recurring feature of an anarchic world.

This is also why classical realists insist that principles are subordinate to policies. A state cannot afford to let moral ideals override its survival. Leaders must constantly adapt to shifting configurations of power, abandoning old commitments and forming new ones as circumstances demand. Flexibility, not loyalty to abstract values, is the mark of effective statecraft.

Managing conflict: diplomacy, balance of power, and law 🔗

It would be a mistake to think classical realism is simply a celebration of war. The theory is pessimistic about human nature, but it is deeply concerned with how to prevent that nature from producing constant catastrophe. Classical realists rely on several traditional tools to manage the perpetual struggle for power and maintain a workable order.

Traditional diplomacy 🔗

Diplomacy sits at the heart of realist conflict management. Through negotiation, dialogue, and compromise, states can resolve disputes without resorting to force. Open communication helps prevent misunderstandings from spiralling into war, while treaties and agreements establish mutually acceptable terms. Classical realists prize the skilled diplomat who understands power, reads other states accurately, and secures the national interest through patient bargaining rather than reckless confrontation.

Balance of power 🔗

The balance of power is perhaps the most distinctive realist mechanism for maintaining stability. The idea is that peace is best preserved when no single state or alliance grows strong enough to dominate the rest. When power is roughly balanced among major states, each is restrained because aggression invites retaliation from others.

For classical realists, this balance does not happen automatically. Morgenthau argued that it is the result of conscious effort by statesmen who form alliances, make strategic decisions, and adjust their commitments to prevent any rival from becoming too powerful. The nineteenth-century European order is the classic example, where shifting alliances among the great powers kept any one country from dominating the continent. Crucially, the system worked best when alliances remained flexible. When they hardened into rigid blocs before the First World War, the balance lost its ability to adjust and collapsed into catastrophe.

International law and its limits 🔗

Classical realists also recognise a role for international law in managing conflict, but they treat it with caution. Law and institutions can facilitate cooperation and provide rules of conduct, yet realists insist they have no independent power to compel states. In the absence of a global enforcer, law works only when it aligns with the interests of powerful states. This is the realist’s honest acknowledgement that rules matter, but power matters more.

Criticisms and lasting influence 🔗

Classical realism is not without its critics. Many argue that its view of international relations is overly pessimistic. The theory struggles to explain sustained cooperation, such as decades of European integration, or the genuine role institutions play in managing trade and arms control. By treating diplomacy and law as secondary to raw power, it may underestimate the very mechanisms that actually prevent many conflicts.

A deeper criticism targets its foundation. The claim of a fixed, power-seeking human nature is, critics say, a philosophical assumption rather than a proven fact. This vagueness about human nature is precisely what later prompted Kenneth Waltz and the neo-realists to relocate the source of conflict from human nature to the structure of the international system itself. Yet even this shift was a refinement of classical realist insights rather than a rejection of them, which is a testament to how deeply the theory shaped the field.

Despite these debates, classical realism endures. Its emphasis on power, national interest, and the limits of morality continues to inform how leaders and analysts read a competitive and uncertain world. From great-power rivalry to regional security competition, the questions Thucydides, Machiavelli, and Morgenthau raised remain strikingly relevant.

What do you think? Does the classical realist claim that human nature is fixed and power-hungry hold up in a world of growing economic interdependence and international institutions? And if states truly act only in their self-interest, is genuine cooperation ever more than a temporary balance of power waiting to break?

References

1. https://en.wikipedia.org/wiki/Classical_realism_(international_relations)
2. https://www.ebsco.com/research-starters/diplomacy-and-international-relations/balance-power
3. https://www.e-ir.info/2019/01/10/an-ethical-dilemma-how-classical-realism-conceives-human-nature/
4. https://www.tandfonline.com/doi/abs/10.1080/10803920903136247
5. https://www.yourarticlelibrary.com/international-politics/morgenthaus-realist-theory-6-principles/48472
6. https://en.wikipedia.org/wiki/Realism_(international_relations)
7. https://fiveable.me/theories-of-international-relations/unit-1/balance-power-theory/study-guide/jPJQHArme2FhHkuw
8. https://fiveable.me/theories-of-international-relations/unit-1/classical-realism/study-guide/Y8PBELPmk81qlShn