Peace and War: A Theory of International Relations
Raymond Aron’s Peace and War: A Theory of International Relations (originally Paix et guerre entre les nations) is a systematic study of international politics organized around a single foundational insight: relations among states take place in a condition of nature, not in the civil state that governs life within political communities. No common sovereign stands above the nations, no authority holds a monopoly of legitimate force, and the possibility of war is therefore never eliminated. From this starting point Aron builds an extended analysis that moves through theory, sociology, history, and normative judgment, refusing throughout to reduce international life either to a closed science or to a simple moral crusade.
The book is divided into four major sections — “Theory,” “Sociology,” “History,” and “Praxeology” — and each performs a distinct task within the overall argument.
Theory
The opening section establishes the conceptual apparatus. Aron argues that international relations can be mapped and that probabilities can be discerned, but that no closed, global “science” of international relations is anything more than a mirage. The dream of a rigorously predictive discipline on the model of economics or physics is, in his view, an illusion, because the subject matter is irreducibly political and contingent. He develops the vocabulary for analyzing what he calls the diplomatic-strategic conduct of states: the ends they pursue (power, security, glory, ideological aims), the means at their disposal (diplomacy, force, economic pressure, subversion), and the structures within which these ends and means interact.
Central to this section is the distinction between types of international systems — notably between homogeneous systems, in which the member states share broadly similar domestic political principles, and heterogeneous systems, in which they do not; and between bipolar and multipolar configurations. Aron considers how these structures shape the likelihood of conflict, the forms of alliance, and the possibilities of cooperation. He also introduces the notion of “universal history”: the process by which the previously separate regional international systems have been drawn together into a single global arena, so that distant peoples now come into regular contact and collision.
Sociology
In the second part Aron turns to the “subpolitical forces” that condition foreign policy. He examines geography, demography, economic resources and organization, the character of political regimes, ideology, national sentiment, public opinion, and the military establishment, asking how each shapes the external behavior of states. He studies the ways in which the domestic order — the constitution of the state, its economy, its social structure — sets limits on and provides resources for its conduct abroad.
But the argument of this section is deliberately anti-determinist. These forces establish the conditions and constraints within which statesmen act, yet they do not dictate the outcome. Aron insists on the “primacy of politics”: the political decision, the exercise of judgment, and the need for prudent statesmanship remain inescapable. He is therefore skeptical of causal theories — economic, geographic, or psychological — that claim to explain foreign policy in full, and he resists the ambition to derive international behavior from any single variable.
History
The third section offers a broad survey of the twentieth century and of the emergence of the “universal history” anticipated in the first part. Aron analyzes the two world wars, the rise of the bipolar order dominated by the United States and the Soviet Union, the extension of the international system through decolonization, and the transformation of strategy brought about by nuclear weapons. He examines the distinctive features of the postwar system: the dialectic between the two camps, the balance of terror, and the interplay of ideological rivalry with considerations of power and interest.
The survey is not narrative for its own sake. It is intended as a demonstration of the analytical framework developed earlier — a test of how history and theory illuminate one another, and evidence for Aron’s view that international relations can be understood only through the reciprocal correction of general concepts by historical particularity.
Praxeology
The final section turns from analysis to norms and conduct. Aron rejects both the “Machiavellian” position that any means are legitimate in the service of the state and the “idealist” illusion that foreign policy can be transcended or abolished. Politics is not a domain in which innocence is available; the statesman must choose among imperfect alternatives, and the appropriate ethic is one of responsibility rather than of pure intention. Aron discusses the morality of war and of the means of warfare, the value and limits of international law and diplomacy, and the meaning of peace.
He distinguishes peace as a state of affairs — the more or less precarious equilibrium of the international system — from peace as an ultimate ideal or goal. He is skeptical of schemes for universal or world government that would purport to abolish the state of nature altogether, and he argues instead that the statesman’s task is to preserve and improve the conditions of a tolerable peace rather than to realize an impossible one.
Overall argument
Across its four sections the book mounts a sustained argument on two fronts: against positivist and behavioralist “scientists” who hope to turn international relations into a predictive discipline, and against idealists who believe politics can be replaced by law or morality. Aron insists on the irreducibly political, contingent, and prudential character of international life, while still affirming that genuine knowledge — of structures, tendencies, and probabilities — is possible and useful. The work draws widely on philosophy, history, economics, and law, and engages figures ranging from Thucydides, Machiavelli, and Clausewitz to Montesquieu, Tocqueville, and Aron’s contemporaries in the study of international politics, including Hans Morgenthau.
The System and Its Varieties
A central conceptual contribution of the book is the analysis of the international system as a diplomatic-strategic configuration. Aron begins from the premise that international relations are relations among political units that recognize no common superior — a domain of plurality and potential violence he describes as a “state of nature” in the precise sense of the absence of a monopoly of legitimate force. Within this domain he distinguishes between homogeneous systems — in which the principal states share a common political regime and set of values, as in Europe after 1815 — and heterogeneous systems — in which they do not, as in the Cold War. The distinction is not merely descriptive: it affects the nature of diplomacy, the stability of the balance of power, and the likelihood that conflicts will escalate. In heterogeneous systems, ideological rivalry tends to make negotiation more difficult, to raise the stakes of every dispute, and to blur the line between legitimate competition and existential enmity.
Diplomacy and Strategy
The book treats diplomacy and strategy as the two fundamental dimensions of international conduct. Strategy is the deployment of force and the management of the threat of force; diplomacy is the conduct of relations by negotiation and other peaceful means. Aron insists that the two are inseparable: the strategic balance conditions what can be achieved diplomatically, and diplomatic alignments shape the strategic situation. His treatment of nuclear strategy — the shift from war as an instrument of policy to a condition in which general war would be a catastrophe no political objective could justify — is a sustained reflection on how the means of violence can outrun the ends of politics.
Sociological Analysis
The book’s sociological section examines the determinants of international behavior — geography, population, economic resources, and the like — but treats them as conditions rather than causes. Aron is interested in regularities — recurring patterns such as the balance of power, alliance formation, and the tendency of systems toward equilibrium or disequilibrium — but he insists that these are tendencies, not laws. History can confirm or frustrate them. This is the methodological middle path of the book: between the nomothetic ambitions of positivist social science and the idiographic particularism that would deny any general knowledge at all.
Method and Significance
The book’s four-part architecture — theory, sociology, history, praxeology — is itself an argument. Each part corrects the others: concepts without history are empty, history without concepts is blind, and neither can substitute for the prudential judgment that belongs to praxeology. In this respect the book is a sustained meditation on the relationship between knowledge and action in international affairs. Its significance lies in the comprehensiveness of its synthesis and in its refusal of the simplifications that dominate both academic and popular discourse about war and peace.
