System-Level Theories

The study of the causes of war in political science has traditionally been dominated by realist theories, which emphasize states' competition for power and security in a high-threat international environment. This chapter summarizes key concepts in realist theories of international conflict, including anarchy, the security dilemma, the spiral model, and the deterrence model. It identifies varieties of realist theories and their hypotheses about the causes of war, which include classical realism, neorealism, defensive realism, offensive realism, and neoclassical realism. After examining balance of power theory in detail, it also addresses hegemonic theories of conflict, including power transition theory, hypotheses on preventive war, and long-cycle theory.

Realist Theories

The realist school of thought traces back to Thucydides' account of the Peloponnesian War between Athens and Sparta in the fifth century BCE. The foundation of realist international theories has been influenced by thinkers like Machiavelli, Hobbes, Rousseau, and several other philosophers and social theorists. After World War I, idealism gained traction, emphasizing international law, but this was challenged by the failure of such efforts to prevent aggression during the 1930s. In this context, Hans Morgenthau's book Politics Among Nations, first published in 1948, led to a revival of realist thinking after World War II.

Realism is not a single theory but rather a constellation of theories that share a common set of assumptions while including distinctive elements. All realist theories maintain that the key actors in world politics are sovereign states (or other territorially defined groups) that act rationally to advance their security, power, and wealth within an anarchic international system.

Realists (and most other international relations theorists) define anarchy in structural terms as the absence of legitimate governmental authority to regulate disputes and enforce agreements between states or other actors. For most realists, anarchy combined with uncertainty about other states' intentions has enormous consequences. It induces insecurity and continuous competition for power, making the international system inherently conflictual.

Political leaders, faced with omnipresent threats, tend to focus on short-term security needs and adopt worst-case thinking. Coercive threats often advance interests, influence adversaries, and maintain reputations. While anarchy does not inherently lead to war, it creates a permissive environment through insecurity, conflicting interests, and international rivalries. Realists typically hold a pessimistic worldview that casts doubt on grand plans for a peaceful international order.

Realists accept that the primary determinant of international outcomes, including wars and peaceful settlements of disputes, is the distribution of power in the international system or within a specific dyad. Thucydides famously stated, "the strong take what they can and the weak suffer what they must." Different conceptions of power and dynamics of power relationships lead to various realist theories that predict distinct outcomes based on particular distributions of power.

Another shared belief among nearly all realist theorists is that wars can arise through deliberate and inadvertent processes; however, different strands of realism may prioritize either path differently. In the deliberate path to war, two states experience a direct conflict of interest, and at least one determines that military force is more likely to achieve its goals than negotiation. Historical examples of deliberate aggression include Hitler’s initiation of the European war in 1939 and Iraq’s invasion of Kuwait in 1990.

In contrast, the second path to war is characterized by status quo-accepting states that inadvertently enter into conflict, often due to the competition for power exacerbated by inherent uncertainties about adversaries’ intentions. If an adversary grows stronger or forms alliances, the uncertainty provokes a desire to enhance one’s own power to remain secure, potentially escalating tensions even when states do not inherently seek conflict. Such defensive measures are often perceived as threats, leading to a security dilemma characterized by action-reaction cycles that can spiral into war.

The Security Dilemma and Conflict Spiral

The security dilemma encapsulates the core of the spiral model of war, wherein states’ actions taken to bolster their security ironically diminish it, leading to cycles of conflict. Noteworthy instances of this phenomenon include World War I and the 1967 Arab-Israeli War, which escalated due to preemptive actions stemming from misguided perceptions. Notably, the Seven Years' War between Britain and France serves as an example where no offensive steps were actively advocated, resulting in a conflict spiral nonetheless.

In contrast to the spiral model, the deterrence model contends that wars arise when deterrence mechanisms fail, specifically when one side lacks the military capability or credibility to respond effectively to aggression. Deterrence theorists typically view predatory behavior as the main avenue to war while downplaying inadvertent processes. They embrace the strategy encapsulated in the adage, "si vis pacem, para bellum" (if you seek peace, prepare for war), advocating military preparedness and aggressive strategies to maintain peace, as opposition can embolden potential aggressors.

Spiral and deterrent theorists critique each other's strategies as worsening war probabilities, with deterrent theorists arguing that conciliatory measures from spiral theorists undermine deterrence and spiral theorists stating that hardline tactics provoke conflict escalation. Both theories falter due to absolute predictions that neglect contextual nuances, raising essential questions about when coercive threats are effective.

Varieties of Realism

Realist international theory functions more as a school of thought than a singular theory, encompassing many variations. One notable distinction is between classical realism and structural (or neorealism) realism, the latter often equated with Waltzian neorealism, which includes defensive and offensive realism. A fifth variant, neoclassical realism, emerged in response to structural realism.

Classical Realism

Classical realists maintained that state behavior is influenced by numerous factors beyond mere anarchic international structures. In addition to the absence of a central authority, they spotlight human nature, suggesting that aggressive instincts, selfishness, greed, pride, and passion drive warfare. Classical realists were invested not only in understanding wars but also in state foreign policies and grand strategies. This approach yielded more detailed theories, albeit less parsimonious than those of their successors.

Waltzian Neorealism

Waltz criticized classical realism for attributing causality to human nature, stating that it fails to explain variations in war and peace across different times and contexts. Thus, he formulated neorealism, which embraces a more rigorous scientific methodology. Neorealism positions the pursuit of security—rather than power—as a primary goal, while emphasizing the importance of international anarchy and the distribution of power among leading states. According to Waltz, the distribution of power significantly influences state behaviors and outcomes, often more than internal state characteristics.

Waltz illustrated that hegemonies are rare in international systems, with balance of power being a common feature of international relations. He asserted that the anarchic nature of the system leads states to emulate successful security practices and that inability to ensure security renders states vulnerable to conquest. Waltz further emphasizes the concept of polarity, arguing that international system structures create distinctive threats and opportunities that inform great power behaviors.

The debate concerning bipolar vs. multipolar systems has seen varied perspectives. While classical realists argued for the stability of multipolar systems due to diverse coalition formation, neorealists like Waltz contend that bipolar systems exhibit stability due to clear focus among powers.

Critique of Neorealism

Waltzian neorealism reshaped international relations discourse, necessitating that new theories contend with a realist alternative. Critics, however, argue that despite offering a framework for understanding war's recurrence, it lacks specificity for particular wars or state behaviors. The need for a theory that explains both international outcomes and individual state actions has led to developments in realist theory that integrate additional variables.

Defensive Realism

Defensive realists align with neorealists on the anarchic nature of international systems creating security threats but assert that anarchy does not unavoidably lead to conflict. They argue that if states only seek security, conflicts can be avoided unless aggressive leaders or misinterpretations of threats arise. Their concept of threat includes geographic considerations—proximity and military toxicity affect perceived risks, creating a nuanced lens to assess state behaviors.

Defensive realists propose that technologies contributing to both offensive and defensive capacities significantly influence both the nature of competition and the probability of war.

Though recognizing aggressive states exist, defensive realists maintain that systematic pressures within the anarchic international system do not function in isolation—domestic factors potentially create aggressive agendas.

Offensive Realism

In contrast to defensive realists, offensive realists emphasize the existence of predatory states as a product of systemic structure and uncertainty about adversary intentions rather than merely domestic variables. They argue that states, even those initially setting the status quo, adopt aggressive strategies to enhance security under extreme competitive uncertainties. Mearsheimer suggests that the path to security lies in achieving regional hegemony; however, global hegemony is unattainable due to the challenges of power projection.

Neoclassical Realism

Neoclassical realists recognize the significance of systemic factors while arguing that a transmission gap exists between system structures and state-level actions. They stress the importance of domestic processes and highlight leaders' perceptions as essential to understanding foreign policymaking. While stressing the constraints provided by systemic conditions, neoclassical realists also address domestic dynamics that influence how states mobilize resources for security. This theoretical evolution reflects ongoing dialogues around war, peace, and power.

Balance of Power Theory

The balance of power is a critical concept in international relations, emerging as a central tenet in discussions about state behavior and systemic dynamics. Historical analyses often designate the eighteenth and nineteenth centuries as a