Realism in international relations is a family of theories that emphasises power, security, national interest, competition and the constraints created by the absence of a central world government. Realists generally treat states as the principal actors in international politics and argue that governments must consider their survival and security because no higher authority reliably guarantees their protection.
The international system is therefore described as anarchic in the technical sense that there is no central sovereign authority above states comparable to a domestic government. Realism does not necessarily claim that every international relationship is permanently hostile or that cooperation is impossible. Rather, it argues that cooperation takes place under conditions in which states remain responsible for their own security and must consider how agreements affect their relative capabilities.
Realist approaches therefore concentrate on questions such as power distribution, deterrence, alliances, military capabilities, strategic competition and national interests. The Stanford Encyclopedia of Philosophy describes realism as stressing the competitive and conflictual dimensions of international politics and identifies states, security, national interests and power as central features of the tradition.
The intellectual roots of realism are commonly associated with thinkers including Thucydides, Niccolo Machiavelli and Thomas Hobbes, although the modern academic tradition developed substantially during the twentieth century. Thucydides is frequently studied because his account of the Peloponnesian War examines the relationship between power, fear, interests and justice. Machiavelli is associated with attention to political necessity and practical statecraft, while Hobbes provided concepts concerning insecurity and the absence of a common superior authority that later influenced realist thinking.
Twentieth century classical realism developed particularly strongly between the First and Second World Wars and after 1945, with figures such as E H Carr and Hans Morgenthau challenging approaches they considered excessively optimistic about international cooperation. Realism subsequently diversified. Classical realism often incorporated assumptions about human nature, prudence, power and political judgement, while neorealism or structural realism associated with Kenneth Waltz concentrated more heavily on the structure of the international system and the distribution of capabilities among states.
Classical realism and neorealism are important distinctions. Classical realists generally give greater attention to human nature, political judgement, national interest and the practical use of power. Morgenthau's realism, for example, treats national interest in terms of power while also discussing prudence and the moral difficulties of political action. Neorealism, especially Kenneth Waltz's structural realism, attempts to explain state behaviour primarily through the structure of the international system rather than through assumptions about individual leaders or human nature.
In Waltz's framework, international anarchy and the distribution of capabilities create incentives for states to behave in ways oriented towards survival. Other realist variants include defensive realism, which argues that excessive expansion can generate counterbalancing and insecurity, and offensive realism, which places greater emphasis on the incentives for powerful states to maximise their relative power. These differences show that realism is not a single doctrine and that statements about what realists believe must specify which realist tradition is being discussed.
The realist concept of anarchy does not mean that international politics is chaotic or completely lawless. It means that there is no overarching sovereign government with general authority over all states. States consequently operate in a self help environment in which they cannot assume that another actor will permanently guarantee their survival.
This condition can create the security dilemma, in which measures taken by one state to increase its security can appear threatening to others, prompting them to increase their own military capabilities. Arms competition can therefore occur even when each participant claims defensive intentions. Alliances can arise as states seek to balance against threats or strengthen their security. Realists also distinguish absolute gains from relative gains.
Cooperation may produce benefits for all participants, but a state can become concerned if another state gains substantially more because the resulting change in capabilities may affect future strategic competition. These concepts provide realism with mechanisms for explaining why cooperation can be difficult even when mutually beneficial arrangements appear economically rational.
Power is central to realist analysis, but power does not mean only military force. Material capabilities can include population, territory, economic production, industrial capacity, technological resources, military forces and strategic geography. Realists differ over which capabilities matter most and over how states convert resources into effective influence. A country with large economic resources may not automatically possess equivalent military or diplomatic power because mobilisation, institutions, alliances, geography and technology affect the conversion of resources into capabilities.
Realists therefore examine the distribution of capabilities across the international system and how shifts in that distribution affect strategic behaviour. Balance of power is another major concept, describing situations in which states or coalitions seek to prevent one actor from acquiring overwhelming dominance. Balancing can involve internal efforts such as military expansion and external efforts such as alliances. Realist explanations consequently treat international order as partly dependent on strategic calculations made under conditions of uncertainty.
Realism has a distinctive approach to international cooperation and international institutions. Realists do not necessarily deny that states cooperate, but they often question whether institutions can fundamentally remove the underlying security problems created by anarchy. International institutions may facilitate cooperation by reducing transaction costs, providing information or coordinating expectations, but realist analysis asks whether powerful states will comply when vital security interests are at stake.
Some realist arguments also emphasise that institutions can reflect the distribution of power rather than existing independently of it. Liberal institutionalists challenge this by arguing that institutions can enable repeated cooperation and allow states to pursue mutual benefits even under anarchy. This disagreement is central to the realist and liberal debate. Contemporary realism therefore should not be reduced to the claim that institutions are irrelevant. Instead, it asks how far institutional rules can constrain states when those rules conflict with security interests or major changes in the distribution of power.
Realism has several important limitations and has been criticised by liberal, constructivist, critical and other approaches. Critics argue that state behaviour cannot always be explained by material power because identities, domestic institutions, economic interdependence, international law and norms can affect preferences and behaviour. Realism can also struggle to explain why states sometimes accept institutions that constrain their immediate freedom of action or why relationships between former enemies can become deeply cooperative.
Constructivists argue that interests and identities are socially constituted, while liberal approaches emphasise institutions, domestic political structures and interdependence. Realism has also been criticised when its assumptions are treated as universal rather than historically contingent. The end of the Cold War generated particular debate because some versions of structural realism had difficulty explaining major systemic transformation. These criticisms do not make realism irrelevant; instead, they demonstrate why contemporary international relations contains competing theories that highlight different causal mechanisms and levels of analysis.
The continuing significance of realism lies in its systematic attention to security, uncertainty, strategic competition and power. It is particularly relevant to analysing military alliances, deterrence, arms races, great power competition, territorial disputes and shifts in the distribution of capabilities. At the same time, realism is not a prediction that war is inevitable. Classical realists frequently emphasised prudence and the avoidance of unnecessary conflict, while structural realists can explain stability through balancing and deterrence.
Nor does realism require the belief that morality has no relevance to international affairs. The Stanford Encyclopedia of Philosophy explicitly distinguishes classical realism from more extreme versions that treat all state interests as automatically justified and notes that classical realism can recognise ethical questions while criticising moralism detached from political circumstances. Realism is therefore best understood as a diverse theoretical tradition that foregrounds security, power and national interest while generating continuing debates about how much those factors can explain in a complex international system.
