From Sparta to Kiev the song remains the same

Why do wars begin? It sounds like one of the simplest questions in history, because wars normally have identifiable beginnings. An army crosses a frontier, a government issues an ultimatum, an alliance is activated or negotiations collapse. Yet knowing how a war started is not necessarily the same thing as understanding why it happened, and almost two and a half thousand years ago the Athenian historian Thucydides made precisely this distinction when he tried to explain the origins of the Peloponnesian War.

It is one reason I think Thucydides remains extraordinarily useful today. We should not read him because Athens was somehow ancient America, or because Sparta was Russia, China or Iran. Historical analogies become rather silly when pushed that far. We should read him because he was interested in something much more enduring: what happens when states that are individually behaving in ways they regard as rational collectively create a situation in which war becomes increasingly difficult to avoid.

The Peloponnesian War began in 431 BC and eventually pitted two very different systems of power against one another. Athens possessed the great naval empire of the Greek world, while Sparta was the dominant land power and leader of the Peloponnesian League. Athens had emerged enormously strengthened from the Persian Wars earlier in the century. The Delian League, originally created as an alliance against Persia, had gradually become something much closer to an Athenian empire. Athens controlled tribute, possessed formidable naval power and intervened increasingly in the affairs of its allies.

None of this meant that Sparta was suddenly weak. That is important, because international politics is frequently about relative rather than absolute power. Sparta could remain extremely powerful while simultaneously becoming less secure if Athenian power was growing faster. From the Spartan perspective, therefore, the question was not simply what Athens could do in 431 BC, but what Athens might be able to do ten or twenty years later if its expansion continued.

Thucydides gives us several immediate causes of the crisis. There was a dispute involving Corinth and its former colony Corcyra, into which Athens became involved. There was the revolt of Potidaea, which was an Athenian subject but had important connections with Corinth. There was also the Megarian Decree, through which Athens excluded Megara from markets within the Athenian empire. Corinth increasingly demanded that Sparta act against Athens, and eventually the Spartan alliance concluded that Athens had broken the existing peace.

If we stopped there, we could construct a perfectly plausible explanation of the Peloponnesian War from the sequence of events immediately preceding it. Thucydides, however, famously tells us that the underlying cause was different. It was, in his judgement, the growth of Athenian power and the fear this produced in Sparta.

That sentence is probably one of the foundational statements of International Relations, although it was written more than two thousand years before International Relations existed as an academic discipline. It shifts our attention away from the incident which starts a war towards the structure within which the incident occurs.

Imagine two rooms containing exactly the same spark. In one there is nothing particularly combustible and the spark goes out. In the other, the floor is covered in petrol. If we want to understand the resulting fire, identifying the spark is necessary, but it is not sufficient. We also need to know why the room had become so combustible.

This is where Thucydides becomes particularly interesting for the modern world, because one of the central problems he identifies is what we would now describe as the security dilemma. States attempt to make themselves safer, but the measures they take can make other states feel less safe. Those states respond by strengthening themselves, which in turn confirms the original state’s suspicion that it really was threatened.

The difficulty is that the same action can therefore look completely different depending upon which side of the frontier you are standing.

Athens could regard the maintenance of its empire and navy as essential to its security. Sparta could regard precisely the same Athenian navy and empire as evidence that the balance of power was moving dangerously against it. Neither side needed to be pretending. Both could genuinely believe that they were responding defensively to the behaviour of the other.

This is an extraordinarily useful idea when thinking about Russia and Ukraine.

There is a temptation in discussing the origins of the Ukraine war to assume that explaining Russia’s perception of NATO somehow means justifying the Russian invasion. It does not. Russia chose to invade a sovereign state, and explaining the strategic environment in which that decision was made neither removes Russian agency nor changes the legal responsibility for the invasion.

But if our question is why the European security environment became so dangerous, then we have to be capable of holding two perspectives in our heads simultaneously.

From the perspective of Poland, Estonia, Latvia and Lithuania, NATO enlargement was a response to insecurity. These states had extremely good historical reasons for fearing Russian power, and joining NATO dramatically increased their security. From the Russian perspective, however, the military alliance which had confronted the Soviet Union during the Cold War was moving progressively eastwards.

The interesting point is that both perceptions can coexist. Indeed, that is precisely what makes a security dilemma so dangerous. The Baltic states wanted NATO membership because they feared Russia. Russia claimed to fear NATO enlargement because NATO was acquiring members closer to Russian territory. Russian attempts to prevent neighbouring countries moving towards the West then made those countries even more afraid of Russia and therefore even more interested in Western protection.

Ukraine became caught inside this process. The more threatening Russia appeared, the stronger the Ukrainian incentive to move towards Western institutions became. Yet the more Ukraine moved towards those institutions, the more threatening Moscow claimed the Western presence had become.

The result was not that war became inevitable. That is where I think the fashionable idea of a “Thucydides Trap” can actually mislead us. Structure creates pressures, but governments still make choices. Putin chose war in 2022. Other choices were available. Thucydides himself does not give us a mechanical theory in which one rising power and one established power automatically fight each other. His account is much richer because political leaders, domestic factions, alliances, perceptions and individual crises all matter.

We can see something similar when we turn to Iran.

Iran looks at the Middle East from a very different strategic position from the United States. American military power extends across the region, Israel possesses overwhelming conventional military superiority and an undeclared nuclear arsenal, while Iran has experienced decades of sanctions, covert operations and repeated discussion of regime change. From Tehran, the development of ballistic missiles, regional proxies and potentially the capability to approach the nuclear threshold can therefore be presented as deterrence.

Washington and Jerusalem see the opposite picture. They see an Iranian state developing missiles, supporting Hezbollah and other armed groups, enriching uranium and extending its influence across the Middle East. Measures which Iran regards as necessary because it is surrounded by enemies are themselves evidence, from the American and Israeli perspective, that Iran is the threat against which they need to defend themselves.

The security dilemma again begins to feed itself. American pressure gives Iran an incentive to acquire additional means of deterrence, while those means of deterrence give America and Israel an incentive to increase pressure. Eventually each side can point to the other’s behaviour as proof that its original fears were correct.

This does not mean that everybody is equally responsible, nor does it mean that all security fears are equally reasonable. International Relations becomes useless if it dissolves into the proposition that everybody has a point. What it does mean is that understanding how your opponent sees the strategic environment is essential if you want to predict what your opponent is likely to do.

There is another aspect of Thucydides which I think is particularly relevant to both contemporary conflicts, and that is the role of alliances.

We tend to remember the Peloponnesian War as a confrontation between Athens and Sparta, but the crisis which produced it involved considerably smaller states. Corinth was central to the disputes over Corcyra and Potidaea, while Megara became another source of confrontation. Sparta had its own allies to consider, just as Athens had to maintain its authority over the Delian League.

This created a problem which remains familiar today. Great powers create alliances because alliances increase their security and influence, but alliances also create commitments. Once a state becomes the leader of an alliance, its behaviour towards a relatively unimportant ally can acquire significance far beyond the intrinsic importance of that ally.

For Sparta, the question was no longer simply whether a particular dispute involving Corinth justified a major war with Athens. It was also what repeated Spartan inaction would do to Sparta’s reputation among its allies. If Sparta could not protect the members of the Peloponnesian League against increasing Athenian power, why should those states continue to regard Sparta as their leader?

The argument therefore moves from immediate interests towards credibility.

That matters enormously in Ukraine. Ukraine is not a member of NATO, and Western governments are consequently not bound by Article 5 to defend it. Nevertheless, once the United States and European states had invested enormous quantities of military equipment, money and political capital in preventing a Russian victory, the consequences of the war became connected to the credibility of Western power more generally.

A Russian victory could affect calculations in Warsaw or Tallinn, but potentially also much further away. Governments in Beijing, Taipei, Tokyo or Seoul observe how American commitments function even when the circumstances are very different. A conflict can therefore acquire importance not simply because of the territory being fought over, but because leaders believe other states will draw conclusions from its outcome.

This is exactly the kind of process Thucydides describes. The issue immediately under dispute becomes connected to a much wider question of reputation, and once reputation becomes involved, compromise becomes more difficult.

The Middle East is even more densely entangled. Iran has historically projected influence through Hezbollah, the Houthis and allied militias, while Israel’s security is closely connected to American power. Gulf states may fear Iranian dominance while simultaneously fearing the consequences of a major American-Iranian war. Actions involving relatively small actors can therefore pull much larger powers towards confrontation.

This is where the simple idea that alliances deter war becomes inadequate. They certainly can deter war, because attacking a small state becomes more dangerous if a great power has promised to defend it. But alliances can also transmit crises. A dispute which might otherwise remain local becomes a test of whether a great power’s promises are credible.

And credibility is a peculiar commodity in international politics because states can end up fighting for it even when the original issue is of relatively little importance.

This helps explain one of the most puzzling questions about war. Why do states not simply compromise when the cost of fighting is obviously greater than the value of the thing they are fighting over?

Part of the answer is that governments are rarely negotiating only about the immediate issue. They are negotiating about what the other side will infer from their behaviour.

Suppose Athens makes a concession to Sparta. Athens may worry that Sparta will conclude that pressure works and demand another concession later. Suppose Sparta backs down. It may worry that Athens and Sparta’s own allies will conclude that Spartan threats are meaningless.

A perfectly sensible compromise over today’s dispute can therefore appear dangerous because of what it might imply about tomorrow’s dispute.

This problem is clearly visible in Ukraine. A territorial compromise could stop enormous human and economic destruction. Yet Ukraine has to ask whether territorial concessions would create peace or merely demonstrate that military aggression can successfully alter borders. If Russia were able to regroup and attack again later, then a settlement which looked rational in the short term might make Ukraine less secure in the long term.

Russia, meanwhile, has attached enormous political significance to the war and to its stated objectives. Even where compromise might be strategically advantageous, Russian leaders have to consider what retreat would imply about Russian power and about the sacrifices already made.

The problem is compounded because neither side can easily make a credible promise about the future. Russia can promise not to invade Ukraine again, but Ukraine has obvious reasons for doubting such a promise. Western governments could make commitments about Ukraine’s future military status, but Moscow may doubt whether those commitments would survive changes of government over the following twenty years.

Political scientists describe this as a commitment problem, and it is one of the most important explanations for why wars can continue even when both sides would prefer some negotiated outcome to continued fighting.

Iran faces an analogous problem over its nuclear programme. The United States wants Iran to accept restrictions which make the development of a nuclear weapon more difficult. Iran, however, has to ask what it receives in exchange and whether any American promise will endure. The experience of the 2015 nuclear agreement and the subsequent American withdrawal inevitably affects Iranian calculations about the value of future commitments.

Conversely, Washington and Israel have to ask whether Iranian concessions represent a durable strategic choice or merely postpone the problem until circumstances become more favourable to Tehran.

Both sides therefore want the other to surrender leverage while being reluctant to surrender their own.

And this brings us back to Thucydides, because beneath the military calculations in his history is something deeply human. States care about fear and interest, but they also care about honour, prestige and how others interpret their behaviour. Governments are not computers maximising security from perfect information. They make decisions through political institutions, with incomplete knowledge of their opponents’ intentions, while worrying about domestic opinion and the judgement of allies.

This creates enormous opportunities for misperception.

One side signals resolve because it wants to deter war. The other side interprets the signal as evidence of aggressive intent and prepares for war. Those preparations are then observed by the first side, which concludes that its fears were justified.

Each individual decision may make sense. The cumulative result can be disastrous.

That, for me, is a far more interesting lesson from the origins of the Peloponnesian War than the popular claim that rising and established powers are somehow destined to fight.

The famous formulation associated with Thucydides is that the growth of Athenian power and the fear it caused in Sparta made war inevitable, although the precise meaning of his judgement continues to be debated. But the history he actually writes shows us years of political choices, diplomatic disputes and opportunities for compromise.

Athens and Sparta were not railway trains travelling towards one another on the same track. Political decisions progressively narrowed the space in which compromise could occur.

And something else happens once war begins.

The war itself changes the problem.

Athens in 431 BC was extraordinarily confident. It was wealthy, culturally brilliant and protected by naval supremacy. Pericles believed that Athens could avoid major land battles, shelter its population behind the Long Walls and use its maritime empire to outlast Sparta.

But wars do not remain confined to the assumptions with which governments enter them. The plague devastated Athens and killed Pericles. Political leadership changed. The conflict expanded geographically. Violence intensified, and eventually Athens undertook the disastrous Sicilian Expedition, committing enormous resources to a campaign which had very little to do with the disputes over Corcyra, Potidaea or Megara which had preceded the outbreak of war.

This is another profoundly contemporary lesson.

Wars create new reasons for continuing themselves.

After years of conflict there are occupied territories which did not exist at the beginning, populations which have been displaced, soldiers who have died, infrastructure which has been destroyed and political constituencies whose careers or legitimacy have become attached to victory. Leaders who might once have accepted a compromise must explain why earlier sacrifices were necessary if they now accept something similar.

The Ukraine war of 2026 therefore cannot simply be solved by reconstructing the diplomatic arguments of February 2022. The war has itself transformed Ukraine, Russia and Europe. Finland and Sweden joined NATO. European defence spending changed. Russia’s economic and strategic relationships shifted. Hundreds of thousands of people have been killed or wounded and millions displaced. Whatever settlement eventually emerges has to deal with the world produced by the war, not merely the world which existed before it.

The same is true of the confrontation with Iran. Every strike, assassination, sanctions regime and proxy confrontation changes the political environment in which the next negotiation takes place. Actions intended to solve today’s security problem can manufacture tomorrow’s.

So what can we actually learn from Thucydides?

I do not think the answer is that history repeats itself. That is much too easy. America is not Athens, Russia is not Sparta, and Iran certainly is not Corinth. Nuclear weapons, international law, global markets and modern communications create a strategic environment which an Athenian of the fifth century BC could scarcely have imagined.

What survives is not the historical situation but the underlying problem.

States live in a world in which they can never be completely certain of one another’s future intentions. They therefore seek security through military power and alliances, but those attempts at security can frighten other states. Governments then have to demonstrate credibility to allies and resolve to opponents, but demonstrations of resolve can make compromise appear humiliating. Agreements which could end conflicts are difficult because each side fears that concessions made today will be exploited tomorrow. Meanwhile domestic politics and human psychology mean that leaders are not simply calculating material interests but reputation, prestige and the political consequences of appearing weak.

The uncomfortable conclusion is that wars do not require irrational actors.

Sometimes the tragedy is precisely the opposite.

Athens could construct a rational argument for maintaining its empire. Sparta could construct a rational argument for resisting the growth of Athenian power. Corinth could rationally demand protection from its alliance. Each actor could pursue what appeared to be its own security, and the interaction between those decisions could make everybody dramatically less secure.

That is the insight I would carry from 431 BC to Ukraine and Iran.

When we watch modern diplomacy, we tend to ask what concessions each side should make. Thucydides suggests that the deeper question is what concessions each side can make without believing that it has endangered itself by doing so.

A durable Ukrainian settlement therefore has to do more than divide territory or stop shooting. It has to create a security structure in which Ukraine does not believe that peace merely gives Russia time to prepare another invasion, while somehow addressing the Russian perception that Ukraine’s security arrangements are themselves threatening.

Any durable settlement involving Iran has the same underlying difficulty. America and its allies need confidence that Iranian restraint is real, while Iran needs to believe that surrendering military or nuclear leverage will not simply leave it more vulnerable to coercion later.

Those are extraordinarily difficult problems because international politics has no supreme authority capable of guaranteeing that everybody will keep their promises forever.

Thucydides understood that world because ancient Greece had no such authority either.

Perhaps that is why a war fought with triremes and hoplites still feels unexpectedly modern. The weapons have changed beyond recognition, but the strategic problem has not. A state tries to become safer, its neighbour becomes frightened, alliances turn local disputes into tests of credibility, compromise begins to look like weakness and eventually somebody decides that fighting now is less dangerous than waiting.

The tragedy is that they may all discover afterwards that they were wrong.

And that is why the origins of the Peloponnesian War still matter. Not because Thucydides gives us a formula for predicting the next war, but because he forces us to distinguish the event which starts a conflict from the political structure which made that event so dangerous in the first place.

If we want to understand Russia and Ukraine, or America and Iran, looking for the next spark is not enough.

We also have to ask who has been pouring the petrol on the floor…in these cases, for me, the answer is clearly NATO, EU and USA. For you?