The history of the most disastrous war in ancient Greek history shows that Hunt is right to worry. As the historian Thucydides made clear, the Peloponnesian War between Athens and Sparta 2,500 years ago broke out not because these two great powers intended it to start, but because actions taken by their allies produced unintended consequences that dragged the Athenians and the Spartans into prolonged and destructive conflict.

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Like the United States, these Greek states were more powerful than their allies, but the reality then was the same as it is today: Strength itself does not ensure peace, and security demands collaborating with allies, not simply making decisions out of pride or anger.

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Sparta and Athens both led coalitions of other Greeks that had long been hostile to one another. They periodically came into conflict, but full-scale war was prevented by an international agreement, sworn before the gods, that they would resolve conflicts by negotiation and arbitration, not battle.

But that promise fell apart when the Corinthians, Sparta’s most powerful ally, got into a bitter confrontation with Corcyra, an island city-state founded by Corinth that angered its mother city by refusing to follow its wishes in international relations. Fearing Corinth’s navy, the Corcyreans begged the Athenians for support.

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The Corcyreans’ request provoked heated debate in Athens, where foreign policy was decided by majority vote at large-scale meetings of adult male citizens. These voters recognized the danger to themselves — they all served in Athens’s citizen-militia military. And so, they decided to aid the Corcyreans only with naval resources: They sent warships to help them establish a defensive perimeter to deter attacks by sea, but the Athenians crewing these ships remained under strict orders not to engage in direct fighting with the Corinthians.

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But war is notoriously affected by the unexpected — by “unintended escalation” — and that is precisely what happened next. When the Corinthians blitzed the Corcyreans’ line of ships, the Athenian commanders couldn’t endure seeing their new allies being slaughtered. So, going against their orders, they responded with force.

Humiliated and outraged by this and other instances of Athenian action, the Corinthians demanded that Sparta punish the Athenians, and they threatened to “reconsider” their obligation to the alliance if the Spartans didn’t pursue this revenge.

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The Athenians responded with blunt, even condescending, denials of wrongdoing, doubling down instead on a message celebrating their own military strength and their right to exercise their power over others. They believed their actions had not broken the treaty and peremptorily instructed the Spartans to return to the negotiating table to try to make their case.

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And then it was the Spartans’ time to evaluate how to respond. As both voters and members of an all-male citizen-militia, Spartan men deliberated just as intensely as the Athenians had. But their debate was notably about whether to launch an invasion, not the merits of finding a peaceful solution, as Sparta had sworn to do.

The experienced Spartan general Archidamus urged the Spartans to maintain their integrity before the gods and the other Greeks by fulfilling their treaty obligation to accept the Athenian insistence on further negotiations. He capped off his advice by stressing how much chance matters in war, urging his fellow citizen-soldiers to exhaust every opportunity to forge a deal that would avert conflict. The risks were too high, he insisted, to rush into a war that was going be costly in every sense.

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But at the end of the day, emotion won. Sthenelaidas, a high-ranking Spartan political official, vehemently argued that Athenians were evil and that the Spartans must act immediately to stop the enemy from committing more crimes. He swayed undecided Spartans by making them declare their support or suffer public shaming. These harsh tactics worked, and by majority vote, Sparta began preparing its army to join their allies in taking aggressive action.

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A last-minute effort to avoid war by limiting Spartan demands on Athens ultimately failed. It could not deter the escalating tensions that had been brewing, and a full war broke out.

It took 27 years of unprecedented bloodshed and expense to end the Peloponnesian War. The Spartans unexpectedly won militarily, but this effort weakened them demographically, economically and morally. They soon conceded Persian control over Greeks in Asia Minor and began a long decline from the pinnacle of power to international insignificance.

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The war ended the Athenians’ military and financial preeminence, as well. Their losses proved so expensive that they never again could afford to build a great navy or assert dominant leadership in war or international trade.

In the end, this “accidental war” produced disaster for both sides.

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