Imagine a state that embarks on a suicidal national adventure because of lies and a refusal to listen to experts. Imagine an island nation that destroys itself because its negotiators live in a fantasy land and cannot see the point of view of a more powerful adversary.

Brexit Britain? Well, yes and no.

These are also two of the most notorious (and bloody) episodes in the calamitous war between Athens and Sparta that ripped Greece apart some 2,500 years ago. It’s a conflict that the British would do well to study. There's a useful lesson: Even at the 11th hour, a deal can be done.

Luckily, it’s easy to read up on the war between Athens and Sparta. A veteran of the conflict called Thucydides wrote an account of it, and his book is a masterpiece of political psychology and strategy. The Brexit parallels are stark.

Unfortunately, Thucydides is not widely read in Britain. He’s far more fashionable in America, where he’s obligatory reading for the officers-to-be at West Point and Annapolis. American generals and politicians see him as the go-to guru on the rise and fall of great powers. Over recent years, Athens versus Sparta has been increasingly held up as a case study for China versus America.

Thucydides was no fan of the very direct system of "people power" democracy in ancient Athens.

That’s a shame, as the similarities between Greece in the 5th century B.C. and modern China are shaky. Thucydides is far more useful as an observer of patterns of behavior. He would immediately recognize the firebrands of Trump’s America and Brexit Britain. It is ironic that Steve Bannon in the U.S. and Boris Johnson in the U.K. have declared themselves as devotees of Thucydides, because they are exactly the sort of political operators who appear as the chief villains in his work.

Ultimately, Thucydides' greatest skill lay in picking apart disinformation, demagoguery and democracy in crisis. He would have found Britain in 2019 a familiar place. He'd seen other states hit the self-destruct button.

The Sicilian opening

Thucydides was no fan of the very direct system of "people power" democracy in ancient Athens and took relish in dissecting popular crazes like the Athenian decision to invade Sicily in 415 B.C.

The Sicilian campaign takes us deep into Brexitland. Like the British in 2016, the Athenians had little sense of the scale of the adventure they were undertaking. They were promised that conquering Sicily would be easy and would make them richer. They were told the gutless enemy was disunited and would easily be divided. People who opposed the invasion feared to speak out through fear of being attacked as unpatriotic. The naysayers were cast as enemies of the people.

The doomed general (Nicias), who was finally entrusted with the Sicilian mission, thought the idea was crazy. The lying, philandering aristocratic dandy (Alcibiades) who pushed for an invasion ultimately scarpered and betrayed his country.

Thucydides tells us there were encouraging “but untrue” reports about the piles of money in Sicily’s treasuries and temples — these were the numbers that could have been painted on the side of a big red chariot.

The dashing but deceitful general Alcibiades stressed that the strength of the Sicilian forces was exaggerated and that they would not fight back with the same patriotism as Athenians. Rebels and fifth columnists would join the Athenians. “In all likelihood, they will want to cut separate deals with us,” Alcibiades promised (at least according to Thucydides' account). It is all reminiscent of U.K. ministers like former Brexit Secretary David Davis, promising to have Berlin and Paris fighting each other for trade deals with Britain.

It turned out those internal rifts in Sicily were hyped, and the invasion ended in disaster.

Overall, the topic that clearly fascinates Thucydides is that so much of the pre-invasion bluster was obviously wrong. There were numerous experts who knew the might of Sicily from trade and military expeditions. Athenian soldiers had even been there not so many years before.

But no one cared. Forget the experts. The armada had to sail. Nicias even attempted a “Project Fear” tactic of describing the huge resources that would be needed to make the expedition a success. But the Athenians doubled down and got behind an even bigger expedition. Once the invasion genie was out, it was impossible to put it back in the bottle.

Two years after being launched, the Athenian force was hacked to pieces around the river Assinarus in eastern Sicily in 413 B.C. The prisoners baked to death in fetid quarries or were sold as slaves. The defeat suddenly made Athens look very vulnerable in its war with Sparta, and the democratic system was (temporarily) overthrown two years later.

Massacre at Melos

The most obvious Thucydidean parallel for where Britain is now makes for even unhappier reading than the Sicilian disaster.

The epitome of a failed negotiation occured in 416 B.C. when the haughty Athenian empire (the EU in this case) approached the island of Melos to tell the inhabitants to fall in line with the imperial project and pay tribute. The plucky Brexiteers on Melos (where the Venus de Milo was found) stood up to the imperialists in what is now one of the most famous debates in global diplomatic theory. The problem is that the islanders clung to nebulous hopes about what they would like the world to look like rather than accepting facts. The Athenians were stunned at their lack of realism.

At its heart, the debate is a study in misjudgment. The Melians fundamentally misread their own weakness. They made vague moral arguments and held out airy hopes that their kinsmen in Sparta might come to help. The Melians’ hopes have echoes in the Brexiteers’ optimistic faith in support from the U.S. and the Commonwealth.

Theresa May would be best advised to tell her backbenchers (and senior officials) to go off and read Thucydides.

Negotiations turned fatal because the Melians underestimated the Athenians’ need to make a show of strength to hold their empire together. Finally, the men of Melos were put to the sword, and the women and children sold into slavery.

Of course, the cocksure Athenians got their comeuppance, met disaster in Sicily and lost the war against Sparta. But that came too late for the Melians.

The good news for the Brits is that the Melians had repeated opportunities to bend to the bigger power. They just didn’t take them. Compromise is possible even at one minute to midnight, but only if you bite the bullet and make a realistic appraisal of your own weaknesses. Just hoping that things will turn out how you want them to is not a strategy.

Thucydides himself would be unsurprised that we are still talking about him. He opens his work by observing that human nature doesn’t change much and that his histories will be of value “for all time.”

To survive, Britain needs to acknowledge its Melian tendency to believe it holds more cards than it does. At times, the British speak as if they think they are the Athenians in the debate. U.K. Prime Minister Theresa May would be best advised to tell her backbenchers (and senior officials) to go off and read Thucydides. It could well prove to be the smartest £7.95 they ever spend.

This article is part of POLITICO’s premium Brexit service for professionals: Brexit Pro. To test our our expert policy coverage of the implications and next steps per industry, email pro@politico.eu for a complimentary trial.