Just War Theory in Thucydides’ History of the Peloponnesian War

“The strong do what they can and the weak suffer what they must”


Thucydides’ history of the Peloponnesian Waris generally acknowledged to be an outstanding example of military historiography. The text is an enthralling yet challenging account of the catastrophic impact of the 27-year war between Athens and Sparta during the 5th Century BC. Its sophisticated appraisal of human psychology, the effects of chance, and the impact of internal and external factors on societies at war defies simplistic interpretation. Indeed, in Thucydides’ own words, the text is a “possession for all time”.[2] Analysing Thucydides is further complicated by the cultural distance between ancient Greece and contemporary society. The Greek polis was the epicentre of a unique mix of political, religious, and economic customs.

How do we draw lessons from the text when multiple perspectives are debated, and the author’s voice is deliberately remote? Given the divergent cultural context, how can we determine just action when declaring and prosecuting warfare? This paper examines military ethics in the context of the Peloponnesian War through the lens of Thucydides. It will outline the ethical code of ancient Greek warfare using the two components of just war theory: jus ad bellum and jus in bello.

Military Ethics and Just War Theory

Military ethics is a wide-ranging and complex term that can relate to anything from administrative military actions in peace time through to the conduct of war. Cook and Syse define military ethics as “professional ethics” and a “critical assessment of LOAC [the laws of armed conflict]”.[3] Some distinguish between social values (morality) and individual decisions (ethics)[4] while others distinguish between moral conduct defined by society (morality) and philosophical reflection on the nature of right and wrong (ethics).[5] A third line of enquiry accepts morality and ethics as interchangeable terms that refer to ‘good’ people and ‘right’ actions.[6] To help define military ethics, this paper draws on just war theory.

Just war theory is a classical-Christian theory, primarily derived from the writings of Aristotle, Cicero and Augustine.[7] The theory outlines a set of mutually agreed rules of combat that evolve between two culturally similar enemies. It comprises two components: jus ad bellum (‘justice of war’) and jus in bello (‘just and fair conduct in war’). Analysing how these concepts are applied in ancient Greek warfare and drawing out distinctions between contemporary and classical military ethics can help to contextualise Thucydides’ history.

Jus ad bellum refers to just declaration of war. It comprises six principles: just cause, last resort, proper authority, right intention, reasonable chance of success, and proportionality. The just cause principle requires a just reason for declaring war and the last resort principle implies that all other solutions have been attempted. The proper authority principle stipulates that a state sovereign power must declare war while the right intention principle implies that war is waged for justice, not for self-interest. The reasonable success principle prohibits engaging in unwinnable wars and, finally, the proportionality principle demands that the desired end-state is proportional to the means by which it is achieved. Jus ad bellum principles concern “the decisions and actions of groups or individuals of greater status”, such as heads of state, the legislature, and inter-governmental organisations.[8]

Jus in bello refers to just action during warfare and comprises the two principles of discrimination and proportionality. The discrimination principle restricts the application of lethal force to the killing of combatants. The proportionality principle, as with jus ad bellum, deems that offensive actions should be proportional to the desired end state. The principles of jus in bello ensure that regardless whether or not the cause is just, military action is constrained by just principles. Consequently, it refers to the actions and decisions by “groups and individuals of lesser status”, such as armies, commanders and soldiers.[9]

Jus ad Bellum

Thucydides famously declared that “the growth of the power of Athens and the alarm which this inspired in Sparta, made war inevitable.”[10] This conclusion provides an overview of the balance of historical forces but does not explain the precise reason for commencing hostilities. Book 1 clarifies that Sparta was led unwillingly into the war by Corinth, another power in the Peloponnesian League. At the time that the war between Athens and Sparta commenced, the Greek city-states were operating under the 446 BC Thirty Year Peace Agreement. The peace agreement identified the members of the Athenian-led Delian League and the Spartan-led Peloponnesian League and stipulated that neither league could recruit allies from the other. Nonetheless, disputes about unaligned cities occurred frequently. Thucydides’ account of the declaration of war suggests that ancient Greek military ethics subscribe to the jus ad bellum principles of just cause, last resort, proper authorities and reasonable chance of success. The principles of right intention and proportionality, however, are absent.

Just cause for declaring war is at the centre of debates between the various combatants. The antagonists raise four reasons for war: self-defence, interference with an agreed treaty, protection of colonies, and interference with an ally. Thucydides makes no comment on whether these appeals to custom and law were justified; however, the recurring refrain by different antagonists suggests they were core elements of ethical thinking.

The last resort principle is underlined by the importance of arbitration in resolving disputes between city-states. During the dispute over Epidamnus, both Corcyra and Corinth agreed to submit to arbitration if the other side agreed to withdraw troops. Corcyra condemned Corinth’s refusal to submit to arbitration because “they chose to prosecute their claims by war rather than by a fair trial.”[11] Athens also suggested arbitration to resolve the dispute with Corinth over Potidaea. Archidamus, the Spartan king opposed to the war, encouraged the Spartans to accept Athens’ offer as it would be wrong to attack a party that has offered arbitration. Therefore, taking military action before pursuing diplomatic avenues of dispute resolution was considered unethical conduct.

Thucydides’ history provides detailed evidence that declarations of war were made by the proper authorities. Regardless whether the city-states were democracies or oligarchies, arguments for or against war were made before properly constituted assemblies. Democratic Athens’ decision to support Corcyra was taken during two assemblies held after hearing the views of representatives from both Corcyra and Corinth. Oligarchic Sparta decided whether or not to declare war against Athens during a Spartan assembly.

Reasonable chance of success also impacts just decisions to go to war. Archidamus, for example, counsels Sparta against declaring war because he doubts that Sparta has the military or financial resources to succeed. He provides a detailed appraisal of Athenian resources and strategy to argue that the war will be long and its outcomes unknown. While Thucydides rarely comments on the speeches, he does introduce Archidamus as having the reputation of “a wise and moderate man”,[12] suggesting that Thucydides may have endorsed Archidamus’ arguments.

Where classical and contemporary principles of jus ad bellum diverge is with the principles of right intention and proportionality. Thucydides’ work demonstrates that self-interest was a legitimate consideration when engaging in warfare. When Corcyra requests an alliance with Athens, for example, Corcyra claims “ it is expedient” to offer assistance.[13] The Corinthian delegates urged Athens not to assist Corcyra as “you will act as we have a right to expect of you, and at the same time best consult your own interest.”[14] Athens proudly defends its leadership by appealing to the three motives of  “fear, honor, and interest.”[15]

Proportionality is also absent when considering just declaration of war. When the Athenians recall the Persian sack of Athens, they make no comment on Persian excess, merely highlighting Athenian courage and resolution during an invasion. The moderate Archidamus cautions against war because Athens will likely be desperate after Sparta ravages Attica and, therefore, “increase the difficulty of dealing with them.”[16] In C5th warfare, the means by which the endstate is achieved was therefore a practical, not an ethical, consideration.

Jus in Bello

While proportionality was absent from 5th Century BC Greek ethical thought, discriminating between legitimate and non-legitimate targets was a key consideration. While contemporary warfare restricts legitimate targets to combatants, ancient Greek warfare only protected a select group of non-combatants and sacred sites. Protected status was offered to troops recovering their dead during a truce, battlefield trophy sites, priests, athletes in religious games, surrendered prisoners, religious sites and envoys,[17] but these protections were inconsistently applied. The protected status of both combatants and non-combatants was often challenged, as indicated through the siege of Melos and the Battle of Delium.

Melos was a neutral island that refused to submit to the power of Athens and the Delian League. Athens tried to convince Melos to submit by appealing to their sense of reason and logic, pointing out that “the strong do what they can and the weak suffer what they must.”[18] The Melians replied that “we speak as we are obliged, since you enjoin us to let right alone and talk only of interest”.[19] After Melos surrendered unconditionally to the ensuing siege, Athens put to death the entire male population, enslaved the women and children and recolonised the city. Athens’ response was motivated by considerations of self-interest, not justice. Their actions demonstrate that the protected status of surrendered combatants was recognised but not guaranteed.

Thucydides’ account of the Athenian defeat at Delium in 423 BC highlights that protections for religious sites and fallen bodies were key components of ethical conduct. When the Athenians arrived at Delium, they fortified the Temple of Apollo. After the battle, the victorious Boeotians refused to return the bodies of the Athenian dead until the Athenians had vacated the temple. Both sides accused the other of breaching Hellenic law through impious acts. The Athenians defended their occupation of the temple by claiming that “anything done under the pressure of war and danger might reasonably claim indulgence even in the eye of the god”.[20] The Boeotians refused to grant a truce for the Athenians to collect their dead because it would be “Athenian ground” which was unacceptable while Athens occupied a Boeotian temple.[21] Both the Athenian and Boeotian arguments suggest a flexible attitude to the protected status of religious sites and troops collecting the fallen dead.


Thucydides’ history is a rich exploration of ethical conduct by societies at war. His work demonstrates that there was broad agreement of just action when engaging in warfare but highlights the often-contradictory demands of just conduct and self-interest. As with contemporary ethics, war could only be declared for a just cause, as a last resort, by the proper authority and with reasonable chance of success. Unlike contemporary ethics, right intention and proportionality were not key considerations. Thucydides’ narrative demonstrates that discrimination was a key element in defining ethical action during war. While troops collecting the dead, priests, competitors in sacred games, envoys, religious sites and battlefield trophies had protected status, this protection was inconsistently applied. Conquered civilian populations had weak protections under agreed ethical codes and their fate was most often determined by the interests of the victorious force.


[1] 5.89, Strassler, R (ed.) 1999, The Landmark Thucydides: A Comprehensive Guide to the Peloponnesian War, Free Press. 

[2] 1.22, Strassler.

[3] pp.119–120, Cook, M and Syse, H 2010, ‘What should we mean by ‘military ethics’?’, Journal of Military Ethics, vol. 9, no. 2, pp. 119–122.

[4] Zigon, J 2009, ‘Within a range of possibilities: morality and ethics in social life’, Ethnos, vol. 74, no. 2, pp. 251–276.

[5] Fisher, J 2004, ‘Social responsibility and ethics: clarifying the concepts’, Journal of Business Ethics, vol. 52, pp. 391–400.

[6] Fisher 2004.

[7] Chapa, J, and Blair, D 2016, ‘The just warrior ethos: a response to Colonel Riza’, Journal of Military Ethics, vol. 15, no. 3, pp. 170–186.

[8] p. 46, Tobia, K 2016, ‘The language of war’, The Monist, vol. 99, pp. 40–52.

[9] p. 46, Tobia 2016.

[10] 1.23, Strassler.

[11] 1.34, Strassler.

[12] 1.79, Strassler.

[13] 1.32, Strassler.

[14] 1.43, Strassler.

[15] 1.76, Strassler.

[16] 1.82, Strassler.

[17] Sheppard, S 2005, ‘The laws of war in the pre-dawn light: institutions and obligations in Thucydides’ Peloponnesian War’, Columbia Journal of Transnational Law, vol. 43, pp. 905–926.

[18] 5.89, Strassler.

[19] 5.90, Strassler.

[20] 4.98, Strassler.

[21] 4.99, Strassler.

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