Realism is a major International Relations Theory that emphasises power, security, national interest, competition and conflict under conditions of international anarchy. Realists generally treat states as the principal actors and assume that states must take responsibility for their own survival because no central international authority reliably guarantees their security. Realism contains several variants, including classical realism and later structural realism or neorealism.
Classical realists such as Hans Morgenthau placed greater emphasis on power, political judgment and aspects of human nature, whereas neorealists such as Kenneth Waltz explained international behaviour primarily through the structure of the international system. Realism is often contrasted with liberal approaches that emphasise cooperation, institutions and interdependence, although realists do not necessarily deny that cooperation occurs. The defining question is why cooperation remains difficult and why security competition persists despite international institutions and economic relationships.
International anarchy is fundamental to Realism. Anarchy in this context means that sovereign states operate without a central authority capable of consistently enforcing rules over all of them. Realists argue that this condition creates a self help environment because states cannot safely assume that another actor will protect them indefinitely. Security consequently becomes a central concern, and states may develop military capabilities, alliances or balancing strategies to reduce vulnerability. Anarchy does not mean that states are constantly at war.
It means that the possibility of coercion or conflict cannot be eliminated by a higher sovereign authority. Classical realist and neorealist thinkers differ over the precise causal importance of human nature, but both traditions give significant analytical weight to the security problems generated by the absence of central authority.
Classical Realism has a long intellectual history extending through Thucydides, Machiavelli and Hobbes and into twentieth century writers such as E H Carr and Hans Morgenthau. Thucydides's account of the Peloponnesian War is frequently treated as an important precursor because it examines power, fear, strategic calculation and relations among independent political communities. Hobbes contributed ideas about insecurity and conflict in an environment without common authority. Morgenthau later developed a twentieth century theory in which political behaviour was analysed through interest defined in terms of power. Classical realism therefore contains both historical and modern components, and its thinkers should not be assumed to hold identical doctrines.
National interest is another central realist concept. Realists commonly argue that governments must consider the security and strategic interests of their political communities rather than assuming that moral aspirations alone determine foreign policy. Morgenthau famously conceptualised political interest through power, while other realists have emphasised security, survival and prudent statecraft. Realism does not necessarily claim that morality is irrelevant. Classical realist thought can recognise ethical considerations while maintaining that foreign policy must account for political consequences, power relationships and prudence. The important distinction is between analysing politics through the constraints of power and asserting that states are morally entitled to do anything that benefits them. The latter is stronger than what all realists claim.
Realism also explains international cooperation differently from liberal theories. Realists recognise that states form alliances, negotiate treaties and participate in international institutions, but they ask whether cooperation remains dependent upon underlying power relationships and compatible with security interests. A state may cooperate when it expects benefits, but uncertainty about other states' future intentions can limit trust.
Realists may also be concerned about relative gains, meaning that a state can worry not only about whether cooperation benefits it but whether another state benefits more and thereby improves its future strategic position. This concern becomes particularly important in military and security relationships where today's economic or technological advantage can influence tomorrow's distribution of power.
The balance of power is a major realist concept describing situations in which states respond to concentrations of capability by strengthening themselves, forming alliances or otherwise attempting to prevent one state from becoming overwhelmingly dominant. Balancing can occur through internal measures such as military development or external measures such as alliances. Realist analysis therefore pays attention to distributions of military and economic capabilities, alliance structures, strategic geography and changes in relative power.
Balance of power is not a single mechanical law producing identical outcomes in every case. Different realist scholars disagree over whether balancing is automatic, how states perceive threats and how domestic politics affects strategic behaviour. Nevertheless, power distribution remains a central analytical variable within the realist tradition.
Critics of Realism argue that it can underestimate international institutions, economic interdependence, domestic politics, international law, non state actors, norms and changes in state identities. Liberal approaches argue that repeated interaction and institutions can facilitate cooperation even under anarchy, while constructivist approaches argue that state interests and perceptions are socially constructed rather than fixed. Realism has also been criticised when it treats security competition as more persistent than empirical evidence supports or when it struggles to explain major normative changes. Realists respond in different ways depending on the variant. Some classical realists incorporate history and political culture more extensively, while structural realists deliberately abstract from many domestic variables in order to explain recurring systemic patterns.
Realism is therefore best understood as a broad family rather than a single proposition that states always seek power. Its common analytical emphasis is on sovereign states, international anarchy, security, self help, national interest, power and strategic competition. Classical realism gives greater attention to human nature, political judgment, history and prudence, whereas Neorealism shifts explanatory weight towards the structure of the international system.
The theory remains influential because it provides a framework for analysing war, deterrence, alliances, polarity and great power competition. It should not, however, be used as an assumption that every international outcome is caused by power alone, since different realist theories have different scope conditions and competing International Relations theories identify additional causal mechanisms.
