The Scythians were nomadic horsemen who dominated a vast realm of the Pontic steppe north of the Black Sea, in present-day Ukraine and southern Russia, from the seventh century to the third century b.c. Unlike other ancient peoples who left not a trace, the Scythians continued to haunt and terrify long after they were gone. Herodotus recorded that they “ravaged the whole of Asia. They not only took tribute from each people, but also made raids and pillaged everything these peoples had.” Napoleon, on witnessing the Russians’ willingness to burn down their own capital rather than hand it over to his army, reputedly said: “They are Scythians!”

The more chilling moral for modern audiences involves not the Scythians’ cruelty, but rather their tactics against the invading Persian army of Darius, early in the sixth century b.c. As Darius’s infantry marched east near the Sea of Azov, hoping to meet the Scythian war bands in a decisive battle, the Scythians kept withdrawing into the immense reaches of their territory. Darius was perplexed, and sent the Scythian king, Idanthyrsus, a challenge: If you think yourself stronger, stand and fight; if not, submit.

Idanthyrsus replied that since his people had neither cities nor cultivated land for an enemy to destroy, they had nothing to defend, and thus no reason to give battle. Instead, his men harassed and skirmished with Persian foraging parties, then quickly withdrew, over and over again. Each time, small groups of Persian cavalry fled in disorder, while the main body of Darius’s army weakened as it marched farther and farther away from its base and supply lines. Darius ultimately retreated from Scythia, essentially defeated, without ever having had the chance to fight.

Killing the enemy is easy, in other words; it is finding him that is difficult. This is as true today as ever; the landscape of war is now vaster and emptier of combatants than it was during the set-piece battles of the Industrial Age. Related lessons: don’t go hunting ghosts, and don’t get too deep into a situation where your civilizational advantage is of little help. Or, as the Chinese sage of early antiquity Sun Tzu famously said, “The side that knows when to fight and when not will take the victory. There are roadways not to be traveled, armies not to be attacked, walled cities not to be assaulted.” A case in point comes from the ill-fated Sicilian Expedition of the late fifth century b.c., chronicled by Thucydides, in which Athens sent a small force to far-off Sicily in support of allies there, only to be drawn deeper and deeper into the conflict, until the prestige of its whole maritime empire became dependent upon victory. Thucydides’s story is especially poignant in the wake of Vietnam and Iraq. With the Athenians, as with Darius, one is astonished by how the obsession with honor and reputation can lead a great power toward a bad fate. The image of Darius’s army marching into nowhere on an inhospitable steppe, in search of an enemy that never quite appears, is so powerful that it goes beyond mere symbolism.

Your enemy will not meet you on your own terms, only on his. That is why asymmetric warfare is as old as history. When fleeting insurgents planted car bombs and harassed marines and soldiers in the warrens of Iraqi towns, they were Scythians. When the Chinese harass the Filipino navy and make territorial claims with fishing boats, coast-guard vessels, and oil rigs, all while avoiding any confrontation with U.S. warships, they are Scythians. And when the warriors of the Islamic State arm themselves with knives and video cameras, they, too, are Scythians. Largely because of these Scythians, the United States has only limited ability to determine the outcome of many conflicts, despite being a superpower. America is learning an ironic truth of empire: you endure by not fighting every battle. In the first century A.D., Tiberius preserved Rome by not interfering in bloody internecine conflicts beyond its northern frontier. Instead, he practiced strategic patience as he watched the carnage. He understood the limits of Roman power.

The United States does not chase after war bands in Yemen as Darius did in Scythia, but occasionally it kills individuals from the air. The fact that it uses drones is proof not of American strength, but of American limitations. The Obama administration must recognize these limitations, and not allow, for example, the country to be drawn deeper into the conflict in Syria. If the U.S. helps topple the dictator Bashar al-Assad on Wednesday, then what will it do on Thursday, when it finds that it has helped midwife to power a Sunni jihadist regime, or on Friday, when ethnic cleansing of the Shia-trending Alawites commences? Perhaps this is a battle that, as Sun Tzu might conclude, should not be fought. But Assad has killed many tens of thousands, maybe more, and he is being supported by the Iranians! True, but remember that emotion, however righteous, can be the enemy of analysis.

So how can the U.S. avoid Darius’s fate? How can it avoid being undone by pride, while still fulfilling its moral responsibility as a great power? It should use proxies wherever it can find them, even among adversaries. If the Iranian-backed Houthis are willing to fight al‑Qaeda in Yemen, why should Americans be opposed? And if the Iranians ignite a new phase of sectarian war in Iraq, let that be their own undoing, as they themselves fail to understand the lesson of the Scythians. While the Middle East implodes through years of low-intensity conflict among groups of Scythians, let Turkey, Egypt, Israel, Saudi Arabia, and Iran jostle toward an uneasy balance of power, and the U.S. remain a half step removed—caution, after all, is not the same as capitulation. Finally, let the U.S. return to its roots as a maritime power in Asia and a defender on land in Europe, where there are fewer Scythians, and more ordinary villains. Scythians are the nemesis of missionary nations, nations that obey no limits. Certainly America should reach, but not—like Darius—overreach.