MODERNISTS LIKE to believe that we have entered an entirely new era of armed conflict. To some military thinkers, it's the primordial nature of the terrorists' beheadings, suicide bombings and improvised explosive devices that has marked a completely new form of "asymmetrical warfare" in which the two sides are terribly mismatched.

Others have a different argument. They say it is our own high-tech, computer-enhanced munitions that have reinvented the very nature of conflict into something called "4th-generational war."

But neither argument could be further from the truth. War is like water — its fundamental character remains unchanging precisely because the nature of the humans who fight it is constant over the centuries. True, the pump — the delivery system of flint, arrows, firearms, nuclear bombs, guided missiles and satellite weapons — radically changes the face of battle with each generation. But the essence of war nevertheless stays the same, as we are reminded when we study the distant past.

More than 2,400 years ago, the Spartans fought the Athenians in a bloody 27-year war that nearly wrecked the Greek city-state in its greatest age. Almost every horror we have experienced since 9/11 had a counterpart centuries earlier in that awful Peloponnesian War.

Limb-lopping? The Athenians ordered the right hands of captured Spartan seamen cut off.

Terrorism? On the island of Corcyra, factions burned innocents alive and executed civilians by running them through a gantlet.

Disease and fear of biological attack? The Athenians lost a quarter to a third of their population to a mysterious plague, and they blamed the outbreak on the Spartans.

Roadside executions? The Spartans rounded up 2,000 of their Helot serfs and butchered them all.

Kidnapped diplomats? The Athenians captured Spartan envoys on the way to Persia, ignored their diplomatic immunity, killed them and cast their corpses in a pit.

We recoiled in horror last September when Chechen terrorists stormed a school in Beslan and more than 150 children were killed in a bloody shootout. But in 413 BC, the Athenians unleashed their Thracian mercenaries on the tiny Boeotian town of Mycalessus. The killers slaughtered men, women and children, burst into a schoolhouse and butchered all the students. They even attacked livestock and, according to the historian Thucydides, "whatever living thing they saw."

But the Peloponnesian War not only reminds us of how thin the veneer of civilization is when war, plague or natural disaster rips it off, it also shows that the reasons states fight each other have remained mostly the same over the years.

Thucydides says the Spartans attacked Athens "in fear" of its growing power — the "real" reason despite the numerous pretexts alleged. And the Athenians defended their earlier acquisition of territory on grounds that they took and kept it out of "fear, honor and self-interest."

In our age of sophisticated economics, we tend to look for material causes for wars — land, resources, populations — rather than remembering these age-old emotional urges. But perhaps we could learn from Thucydides the next time Osama bin Laden alleges in his fatwas that we provoked him by stationing troops in Saudi Arabia or by enforcing the U.N. oil-for-food embargo.

The fact is, the deep-seated anger and humiliation of Al Qaeda were more likely incited by a globalized and Western culture that really did threaten all the old hierarchies of an increasingly dysfunctional Arab and Islamic world (and the worried mullahs, patriarchs and theocrats, whose sense of privilege and honor derived from that world).

In other words, Bin Laden probably went to war over a sense of lost honor, in Thucydidean fear of Western globalization and due to his perceived self-interest — given perceptions of Western appeasement of radical Islamist terrorism since 1979 — that he had more to win than lose by hitting New York and Washington.

Of course, we must be careful when evoking the past to make sense of the present. Many, for example, recently cited the Iraq war as the modern equivalent of the disastrous Sicilian expedition of 415-413 BC, when Athens lost most of its fleet by assaulting distant Syracuse. But Syracuse was democratic, larger than Athens and, until the invasion, mostly neutral during the Peloponnesian War. A more historically apt analogy to that expedition would be if the United States had attacked democratic India during the midst of the U.S. war against Al Qaeda.

Study of the Peloponnesian War should also remind us that it is not assured that the wealthiest, most sophisticated and democratic state always triumphs over less impressive enemies. After all, Athens, for all its advantages, finally lost its war. And as Thucydides reminds us about the democratic empire's lapses, arrogance and major blunders, more often the chief culprit was its own infighting and internal discord than the prowess of its many enemies.