Two years into his Italian campaign, Hannibal had crossed the Alps, destroyed one Roman army at the Trebia and another — with its consul — in the fog at Lake Trasimene. Rome’s answer in 216 BC was mass: both consuls, Lucius Aemilius Paullus and Gaius Terentius Varro, took the field together with roughly 80,000 men — by most reckonings the largest army the Republic had ever assembled — to crush perhaps 50,000 Carthaginians, Gauls, Iberians, and Africans on the plain by the Aufidus river in Apulia, near the supply depot of Cannae.
Hannibal arranged his army as a trap in plain sight. The centre — his expendable-looking Gallic and Iberian infantry — bowed toward the enemy in a crescent; his veteran African infantry waited in deep columns at the wings; his cavalry, the one arm where he held clear superiority, took the flanks.
The pocket
On 2 August 216 BC the Roman mass drove into the crescent’s centre, which fought and yielded, fought and yielded — bending backward by design, drawing the legions deeper as their own numbers compressed them into a crowd that could barely raise its shields. Then the jaws worked: the African columns wheeled inward against the Roman flanks, and Hasdrubal’s cavalry, having swept the Roman horse from the field, closed the ring from behind. Rome’s largest army had been converted into a surrounded mob — and the rest of the day was killing, at push of shield and blade, until the sources’ arithmetic: around 50,000-70,000 Roman dead (Polybius says 70,000; Livy 48,000, with tens of thousands captured), including the consul Paullus, both consuls’ quaestors, twenty-nine military tribunes, and eighty men of senatorial rank. Hannibal lost perhaps 6,000-8,000. It stands among the deadliest single days of battle in human history — a rate of killing not matched until the industrial age.
Tradition gives the epilogue to his cavalry commander Maharbal, urging an immediate march on Rome and answering Hannibal’s refusal: “You know how to win a victory, Hannibal; you do not know how to use one.”
The war after the masterpiece
Rome’s response defined Rome: the Senate forbade public mourning beyond thirty days, thanked the surviving consul Varro “for not despairing of the Republic,” declined even to ransom prisoners — and enrolled slaves and debtors into new legions. No peace was discussed. Refusing pitched battle in Italy, Rome bled Hannibal by attrition and struck his bases in Spain and Africa, until Zama (202 BC) reversed Cannae’s choreography against its author.
The battle itself never demobilised: as the model double envelopment — the annihilation of a superior force by a smaller one — Cannae has been the obsession of war colleges from Frederick and Clausewitz to Schlieffen (who titled his collected studies Cannae) and the planners of 1991’s Desert Storm: the perfect battle, and the standing lesson that a perfect battle is not a won war.