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Thucydides and the Morality of Power

Updated: Jul 27, 2023

In a single episode of the Peloponnesian War, Athens slaughtered all Melian men of military age and sold Melian women and children into slavery as part of the government’s political realist foreign policy. Dr. Orton investigates the ancient massacre that defined international relations’ oldest theory.



Political realism – the view that governments’ priority in international relations is power and security for their own state – is an ancient political theory. It was famously set out in the fifth century B.C.E by Thucydides in his History of the Peloponnesian War: “…you know as well as we do that, when these matters are discussed by practical people, the standard of justice depends on the equality of power to compel and that in fact the strong do what they have the power to do and the weak accept what they have to accept.”


This idea is the subject of much debate in fifth century Athens. Political realism finds keen proponents: in Plato’s Republic, Thrasymachus tells the group that that ‘justice or right is simply what is in the interest of the stronger party.’ Today, political realism is a popular way of explaining international relations phenomena, from the 2003 US-led invasion of Iraq to the current war in Ukraine.


Realism, History and Rationality


I’ve written here about the transition away from mythological narrative towards rational accounts in the Greek world in the fifth and sixth centuries B.C.E. and the emergence of political realism is very much a part of this development. History as a discipline was in its infancy, but already historians were reflecting on the rationality of their field. Thucydides’ History of the Peloponnesian War argues for the objectivity of the historian.


Thucydides believes in a rational approach, firmly rooted in evidence, and rejects the mystical. He thinks that it is possible to attain a truth that is not relative to one’s own position, trying to write as though he has no ‘position’ himself: “I have made it a principle not even to be guided by my own general impressions…Not that even so the truth was easy to discover: different eye witnesses give different accounts of the same events, speaking out of partiality for one side or the other or else from imperfect memories.”



The first thing to do, says Thucydides, is to reject the approach of the poets, “...who exaggerate the importance of their themes, or of the prose chroniclers, who are less interested in telling the truth than in catching the attention of their public, whose authorities cannot be checked, and whose subject-matter, owing to the passage of time, is mostly lost in the unreliable streams of mythology.”


Instead, Thucydides is not concerned about producing an account that is romantic or easy to read, but wishes to produce a factual account that is rooted in the evidence. Nevertheless, Thucydides is honest enough to admit that he finds it difficult to remember detail, and that he sometimes finds it necessary to recreate events “…while keeping as closely as possible to the general sense of the words that were actually used, to make the speakers say what, in my opinion, was called for by each situation.”

This respect for rationality goes hand in hand with his rejection of the mystical. Although the intellect can be relied upon to discover the truth, it must be isolated from the memory and emotion. Thucydides knows that people “adapt their memories to suit their sufferings,” and the historian must compensate for this. He remembers an old oracle that predicts: “War with the Dorians comes, and a death will come at the same time.”


Considering the controversy as to whether the word in the ancient verse should be ‘dearth’ rather than ‘death,’ he notes that ‘death’ is the interpretation remembered by the Athenians at the time of the plague, but “Certainly I think that if there is ever another war with the Dorians after this one, and if a dearth results from it, then in all probability people will quote the other version.”



This differs from the approach to oracles and religion taken by earlier historians like Herodotus. Herodotus thinks that the gods do send omens to warn us, and offers his own interpretations of divine signs. Although Herodotus acknowledges that some oracles can be ambiguous or wrong, in many cases, he simply describes the oracular advice without Thucydides’ cynicism.


Politics and Power


In his analysis of domestic politics, Thucydides explains that systems of government are assessed according to the distribution of power: “Our constitution is called a democracy because power is in the hands not of a minority but of the whole people.” War is explained in terms of the occurrence of imbalances of power, with the cause of the Peloponnesian War given as “the growth in Athenian power and the fear which this caused in Sparta.”


The centrality of power in international relations is famously embodied in the Melian Dialogue of Book Five of Thucydides’ History. The Athenians send an expedition to Melos, an island community who refused to join the Athenian Empire and thus became enemies of Athens.


In a desperate attempt to save their lives, the Melians try to appeal to traditional morality. When they ask about the ideas of fair play held by the Athenian subjects, the reply is most telling: “…if we were on friendly terms with you, our subjects would regard that as a sign of weakness in us, whereas your hatred is evidence of our power.”



Power is a principle to which even the strong are bound, regardless of whether or not they choose it. It is as vital for the security of the stronger party as it is for the imposition of their will. The Athenians believe that they cannot afford to ignore the demands of power politics, even if they wish to do so.


Power and the Natural Law


In Prometheus Bound, playwright Aeschylus tells us that even the gods’ political relationships are governed by a higher law. Zeus’ policies are harsh because new power must always be so. In The Persians, Aeschylus further implies that the limits that constrain the gods are even more restrictive in human affairs, because the gods are there to impose them.This is echoed by Herodotus, who points out in the Histories that the gods set limits on human behaviour and that arrogance and ambition can lead to downfall.


Thucydides would have us believe that his political analysis is divorced from morality, which he sees as merely “a great mass of words that nobody would believe.” Note that the Athenians’ response is given in terms of interest, not righteous anger about the Melians’ previous refusal to ally with them. This is a marked contrast to the kind of motivation that Herodotus ascribes to political actors, which frequently revolves around morally-charged arguments like retribution.


People often speak about political realism as though it is the next step from moral relativism. The argument goes that, if there is no universal moral law, there is nothing to stop the powerful from imposing their will on the weak – which will ultimately lead to political realism.


However, I think this is a mischaracterisation. I’ve written here about how moral relativism leads to conservatism in the ancient world, not to political realism. By taking a closer look at Thucydides famous history, we see that political realism is as far removed from moral relativism as it is from the traditional morality that the Melians try to invoke.


The strong are not free to act as they like. There is a Law of Nature, which determines the nature of political relationships. Although knowledge of this Law is to some extent empowering, its existence necessarily constrains even the powerful.


Find Out More


To read about the ancient transition from myth to reason, read Dr. Orton’s blog post The Ship, the Shore and the Philosopher. To lean how moral relativism produced ancient conservatism, read her post The Radical Roots of Ancient Conservatism. If you’d like to join the debate about political theories in the ancient world, we offer adult interest philosophy course in Ancient Philosophy, or our politics courses in the History of Political Philosophy and Political Ideologies.


Alternatively, find out more about political realism in our politics courses, Introduction to International Relations, International Relations Theory or International Relations history. These are templates of possible routes of study and can be combined, adapted, or designed from scratch to suit your interests and goals.


Dr. Orton will work with you to design a course of private tutorials tailored to your needs, ability and schedule – whether you are undertaking your own research for an independent project, writing a book or simply have a personal interest. Find out what it’s like to work with her here.


Contact us to find out more!


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