Introduction
The act of war, representing organised violence on a mass scale, has perpetually raised profound moral and legal questions. The attempt to reconcile the brutal reality of armed conflict with principles of justice and morality has been a central concern for philosophers, theologians, and lawyers for millennia. The central issue is whether war can ever be morally justified, and if so, under what conditions. This essay will examine the evolution of thinking on the morality of war by tracing its conceptual development. It will consider definitions of war and its justification offered by classical thinkers such as Plato, Aristotle, and Cicero, before exploring the formulation of the Christian Just War tradition. Subsequently, the modern realist and idealist perspectives of Thomas Hobbes and Immanuel Kant will be analysed, concluding with an assessment of how these historical ideas inform the modern legal concept of war, especially within the framework of international law.
Classical Perspectives on War
In classical antiquity, war was often viewed as a natural, albeit unfortunate, feature of the human condition and inter-state relations. For Plato, war was an unavoidable consequence of the state’s growth and its need for resources, which would inevitably lead to conflict with its neighbours. In his work The Republic, the ideal state requires a specialised class of 'Guardians' to defend it, suggesting that military readiness is a core function of a well-ordered society. War’s primary justification was the preservation and security of the polis (city-state). However, Plato also distinguished between conflicts among Greeks, which he viewed as a form of civil strife, and wars against 'barbarians', indicating an early conception of rules that might apply differently depending on the identity of the enemy (Plato, 1992).
Aristotle, a student of Plato, also accepted war as a necessary activity, but one that should be conducted for the sake of achieving a lasting peace. In his Politics, he argued that military training and war itself should not be ends in themselves but instruments to secure the state against enslavement and to establish a just order (Aristotle, 1992). The ultimate goal of war was not conquest for its own sake, but the establishment of a state of peace in which citizens could flourish. This view suggests that war must have a rational purpose tied to the well-being of the community.
The Roman orator and philosopher Cicero provided a more structured foundation for the moral justification of war, acting as a crucial link between classical thought and the later Christian tradition. In De Officiis (On Duties), Cicero argued that the only just reason for going to war was either in self-defence or to avenge an injury. He stated that no war is just unless it is waged after a formal demand for restitution has been made or a formal declaration has been issued. This introduced the important procedural and legalistic elements of proper authority and just cause, asserting that war must be a last resort and conducted honourably (Cicero, 1991). Cicero's ideas laid the groundwork for a more systematic framework for evaluating the legitimacy of armed conflict.
The Christian Just War Tradition
The emergence of Christianity presented a challenge to classical views, as its initial teachings emphasised pacifism. However, as the Roman Empire adopted Christianity, theologians had to reconcile the pacifist ethos with the political reality of defending the state. St. Augustine of Hippo was a pivotal figure in this development. He argued that while individuals should 'turn the other cheek', a Christian ruler had a duty to protect the innocent and punish evil, which could necessitate war. For Augustine, a just war was one that sought to secure peace and punish wrongdoing, waged with a 'right intention' of love for the enemy rather than hatred or greed (Augustine, 1998).
This foundation was later systematically developed by St. Thomas Aquinas in his Summa Theologiae. Aquinas outlined three core conditions for a war to be just, which form the basis of the jus ad bellum (justice of going to war) doctrine:
- Proper Authority: War must be declared by a sovereign who has the authority to do so, not by private individuals.
- Just Cause: There must be a just reason for going to war, such as defending the state against attack or redressing a grave injury.
- Right Intention: The intention of the sovereign must be to promote good or avoid evil, with the ultimate goal being the restoration of a just peace.
Later scholars added further conditions, including Last Resort (all peaceful options must be exhausted), Proportionality (the good achieved by war must outweigh the harm it causes), and Reasonable Prospect of Success (a war should not be fought if it is futile). Alongside jus ad bellum, the tradition also developed principles for jus in bello (justice in war), primarily concerning Discrimination (non-combatants must not be targeted) and Proportionality (the force used must be proportional to the military objective).
Modern Philosophical Approaches: Hobbes and Kant
The early modern period saw a departure from the theological framework of the Just War tradition, with philosophers proposing alternative foundations for understanding conflict. Thomas Hobbes, writing in the aftermath of the English Civil War, offered a starkly 'realist' perspective in his 1651 book Leviathan. For Hobbes, the natural state of humanity without a powerful sovereign was a "war of all against all," where life is "solitary, poor, nasty, brutish, and short" (Hobbes, 1996). In this state of nature, there is no justice or injustice, and every person has a right to everything. Security is only possible by surrendering individual rights to an absolute sovereign, the 'Leviathan', whose primary duty is to maintain order and security. In relations between states, which remain in a state of nature with one another, the sovereign has the right to wage war for self-preservation. In Hobbes's view, the morality of war is secondary to the imperative of state security.
By contrast, Immanuel Kant offered a profoundly 'idealist' vision. In his 1795 essay Perpetual Peace: A Philosophical Sketch, Kant argued that morality required humanity to strive for a permanent end to war. He did not believe this was a naive hope but a rational destination. Kant proposed a model based on three "definitive articles" for achieving perpetual peace:
- The civil constitution of every state should be republican (i.e., representative and based on the rule of law), as citizens who bear the costs of war would be reluctant to consent to it.
- The law of nations shall be founded on a federation of free states. This would be a league of nations committed to resolving disputes peacefully.
- Cosmopolitan law shall be limited to conditions of universal hospitality, meaning states should treat foreigners with respect.
For Kant, war is fundamentally irrational and immoral. His work provides the philosophical basis for modern international institutions and the pursuit of a global legal order designed to prevent war (Kant, 1983).
The Modern Concept of War in International Law
The philosophical debates outlined above have profoundly influenced the modern legal framework governing war, known as Public International Law. The Just War tradition, in particular, provides the intellectual architecture for contemporary laws of armed conflict. The UN Charter, for instance, reflects the principle of jus ad bellum. Article 2(4) establishes a general prohibition on the threat or use of force in international relations. This serves as a modern equivalent of the presumption against war. Exceptions to this prohibition are narrowly defined and echo Just War principles. The right to self-defence under Article 51 reflects the 'just cause' of responding to an armed attack, while Chapter VII allows the Security Council to authorise collective action to maintain or restore "international peace and security," functioning as the modern 'proper authority'.
Similarly, the principles of jus in bello are codified in the body of law known as International Humanitarian Law (IHL), most notably in the Geneva Conventions of 1949 and their Additional Protocols. The core IHL principles of distinction, proportionality, and precaution are direct descendants of the Just War requirements to discriminate between combatants and non-combatants and to use only necessary force. These rules attempt to impose moral and legal limits on the conduct of hostilities, even in the brutal environment of the battlefield.
Conclusion
The moral and legal justification of war has been a persistent theme in Western thought, evolving from the classical acceptance of war as a natural function of the state to the sophisticated framework of Just War Theory and the aspirational idealism of Kant. Thinkers from Plato and Cicero to Aquinas and Hobbes have grappled with the tension between the state's need for security and the moral demand for justice. These philosophical and theological debates have not remained abstract. They have shaped the modern international legal order, which attempts to constrain the resort to war and regulate its conduct. While the effectiveness of international law in preventing conflict is often questioned, its core principles, derived from centuries of moral and philosophical inquiry, continue to provide the primary framework through which the international community judges the legitimacy of armed force.
References
Aristotle (1992) The Politics. Translated by T.A. Sinclair, revised by T.J. Saunders. Penguin Classics.
Augustine, (1998) The City of God against the Pagans. Edited by R.W. Dyson. Cambridge University Press.
Cicero, M.T. (1991) On Duties (De Officiis). Edited by M.T. Griffin and E.M. Atkins. Cambridge University Press.
Hobbes, T. (1996) Leviathan. Edited by Richard Tuck. Cambridge University Press.
Kant, I. (1983) 'To Perpetual Peace: A Philosophical Sketch'. In Perpetual Peace and Other Essays. Translated by Ted Humphrey. Hackett Publishing.
Plato (1992) The Republic. Translated by G.M.A. Grube, revised by C.D.C. Reeve. Hackett Publishing.
