For nearly 2,500 years, the Athenian general and historian Thucydides has been a popular source of strategic wisdom in the West. Lately, his name has popped up in Asia, too, especially in reference to U.S.–China relations. “We all need to work together to avoid the Thucydides trap,” said Chinese president Xi Jinping in 2013. Xi was borrowing a phrase coined by Harvard political scientist Graham Allison to describe the challenges “that can emerge when a rising power challenges a ruling power—as Athens did Sparta in Ancient Greece and as Germany did Britain a century ago. Most of these dangerous challenges have ended badly, often for both sides.”

Not everyone agrees. “In the sparest structural sense,” writes Tufts University’s Daniel D. Drezner, “there are a few parallels that can be drawn between Greece in the fifth century B.C.E. and the present day. On the whole, however, I think the Athens-Sparta historical analogy obfuscates more than it enlightens.” James Holmes of the U.S. Naval War College agrees that ancient Sparta and modern America are not terribly comparable. “Far from being an established custodian of the regional order, the Spartans were loath to exercise leadership,” he writes. “That’s different from a Great Britain or an America at its zenith, a global marine power jealous of its standing.” Drezner does concede that “The way in which the Thucydides analogy matters is just how much Chinese and American policymakers think it matters.” And the Thucydidean trinity—“prestige, fear, and self-interest”—seems to be at work in the U.S.–China relationship. That’s why the first book of the History of the Peloponnesian War and its most famous sentence—“In my view the real reason [for the war], true but unacknowledged, which forced the war was the growth of Athenian power and Spartan fear of it”—is so widely quoted today.

I’ve been reading the History of the Peloponnesian War with the class of 2016 of the Institute for Political Studies at the Portuguese Catholic University in Lisbon, and I’d suggest at least five reasons why Thucydides deserves to be studied carefully today. To start, Thucydides helped to invent history as a discipline. The Athenian general clearly believed that understanding the past would be useful for ordinary citizens, decision-makers, institutions and societies. We live today in a time of great change in international politics. Regional orders are under pressure in East Asia, Europe, and the Middle East. Knowledge of history should matter greatly—competent statecraft is impossible without it—but it doesn’t seem to matter at all. In Europe and in the U.S., historical ignorance has become almost a badge of honor.

Thucydides was also a political historian. His masterwork is a biography of two political regimes: the ambitious, confident, and democratic Athens and the oligarchic and conservative Sparta. Which one, Thucydides asks, aspired to greatness, staved off internal disintegration, practiced a prudent and consistent foreign policy, and adapted better to what turned out to be a long war? Thucydides argued that good strategy and foreign policy always began at home. For him, the greatest threat to a dynamic maritime and commercial democracy came from within. His book is an indictment of Athenian direct democracy after Pericles’s death.

Third, Thucydides was the first grand strategist, and he remains one of the best to write about the subject. Grand strategy is the art of using a state’s political, diplomatic, economic, cultural, and military tools to advance its interests. Pericles continued and developed Themistocles’s grand strategy for the rise of Athens. Fortification of the city and its harbor, trade and financial integration with its allies, public works that emphasized excellence, open sea lanes, and a formidable navy were the pillars of Pericles’s vision.

One of the Peloponnesian War’s enduring lessons is that it is very difficult for a continental power to defeat a trading nation with a strong economic base, sea power, and competent leadership. As late as 406 B.C., the Spartan admiral Callikratidas wanted to put an end to what he called Athens’s “illicit love affair with the sea.” Sea access was crucial for Athens, but the city needed to maintain a navy that was second to none. In his final speech, Pericles reminded listeners that “with the naval resources you have at your disposal, no one, neither the King of Persia nor any other nation now on earth, can prevent you from sailing where you will. So this power is clearly of a different order from the utility of houses and land, the loss of which you consider a great deprivation.”

Pericles’ successors forged a different path, however, and the Athenians were roundly defeated by a Sparta financed and allied with the Persian Empire. But did Athens lose in Sicily in 415–413 B.C. because it chose the wrong strategy or because the Athenian general Nicias’s operational command throughout the Sicilian expedition was disastrous? Could Demosthenes, Athens’s most aggressive and tactically savvy infantry general, have achieved victory, or at least saved the city’s army and navy, if he had been in command from the beginning in Syracuse? These questions remain relevant to our discussions of foreign policy and leadership today.

Fourth, Thucydides understood that individuals made a difference—in the city and on the battlefield. Statesmanship, military leadership, and moral virtue play a vital role in the History of the Peloponnesian War. Thucydides watched the tragedies of Sophocles and the comedies of Aristophanes. He saw the stunning Parthenon being built in the Acropolis. He listened to scientists, philosophers, and sophists who were revolutionizing their fields. He lived in a brilliantly creative city and knew personally or wrote about Pericles, Cleon, Nicias, Demosthenes, Brasidas, Alcibiades, Athenagoras, Hermocrates, and Phrynicus—men whose deeds and words remain essential to a liberal education in the twenty-first century. As the Economist’s Adrian Wooldridge wrote recently, “You will learn more about leadership from reading Thucydides’s hymn to Pericles than you will from a thousand leadership experts.” Pericles was the rarest of men—a statesman, a strategist, and a superb politician.

Finally, Thucydides was a tough warrior. He wrote from firsthand experience about the tragedy of war—never more movingly than in his account of the final collapse of the Athenian expeditionary force at the river Assinarus in Sicily in 413 B.C. “I will attend every class, but I will not come to the one when the Athenians die in the river,” a former student told me last year. “That passage was one of the most overwhelming things I read in my life.” A professor of classics told me something similar. “That scene is terrible,” he said. “I had tears in my eyes when I translated those paragraphs. It was as if I could see them all. I felt I was there with them.”

A long, unfinished, and sometimes difficult book about a war in the fifth century B.C. that is still quoted today in the U.S., Europe, and Asia can only be a classic. Most of what we read today will be obsolete by 2030. How many books will survive another century? Only a few. I’m quite sure that the History of the Peloponnesian War will be one of them.