The army of Hannibal, fronting west, stood on 2 August, 216 B.C., in the Apulian plain to the left of Aufidus (Ofanto) in the vicinity of the village Cannae, situated near the mouth of the river, and opposite the troops of Consul Terentius Varro. The latter, to whom had been transferred by the other Consul Aemilius Paulus the daily alternating commandership, had
55,000 heavily armed men,
8,000 lightly armed men,
6,000 mounted men,
on hand and, in the two fortified camps,
2,600 heavily armed men,
7,400 lightly armed men.
___________
10,000 men.
at his further disposition, so that the total strength of the Roman army amounted to 79,000 men.

Hannibal had at his disposition only
32,000 heavily armed men,
8,000 lightly armed men,
10,000 mounted men.
_________
50,000 men.
His position, with a considerably superior enemy in his front and the sea, in his rear, was by no means a favorable one. Nevertheless, Consul Aemilius Paulus, in concurrence with Proconsul Servilius, wished to avoid a battle. Both feared the superior Carthaginian cavalry to which Hannibal particularly owed his victories on the Ticinus, on the Trebia and at the Trasimene lake. Terentius Varro, nevertheless, wished to seek a decision and avenge the defeats suffered. He counted on the superiority of his 55,000 heavily armed men as against the 32,00 hostile ones, consisting of only 12,000 Carthaginians and of 20,000 Iberians and Gauls who, in equipment and training, could not be considered as auxiliaries of full value. In order to give increased energy to the attack, Terentius gave his army a new battle formation.(3) The cavalry was placed on the wings. The lightly armed troops, destined to begin the combat, to envelop the enemy and to support the cavalry, were not much considered by either side.
Hannibal opposed to the enemy's front only his 20,000 Iberians and Gauls, which were probably 12 files deep. The greater part of his cavalry under Hasdrubal was placed on the left wing and the light Numidian on the right. In rear of this cavalry the 12,000 heavily armed Carthaginian infantry were formed equally divided between the two wings.

Both armies advanced against each other. Hasdrubal overpowered the weaker hostile cavalry on the right flank. The Roman knights were overwhelmed, thrown, into the Aufidus or scattered. The conqueror turned the hostile infantry and advanced against the Roman cavalry on the wing which, until then, had only skirmished with the Numidianm light horse. Attacked on both sides, the Romans were here also completely routed. Upon the destruction of the hostile cavalry, Hasdrubal turned against the rear of the Roman phalanx.
In the meanwhile, both infantry masses had advanced. The Iberian and Gallic auxiliary forces were thrown back at the impact not so much on account of the strength of the attack of the 36 Roman files as on account of the inferior armament and the lesser training in close combat. The advance of the Romans was, however, checked, as soon as the Carthaginian flanking echelons, kept back so far, came up and attacked the enemy on the right and left, and as soon as Hasdrubal's cavalry threatened the Roman rear. The triarii turned back, the maniples of both wings moved outward. A long, entire square had been forced to halt, fronting all sides and was attacked on all sides by the infantry with short swords and by the cavalry with javelins, arrows, and slingshots, never missing In the compact mass. The Romans were constantly pushed back and crowded together. Without weapons and without aid, they expected death. Honnibal, his heart full of hatred, circled the arena of the bloody work, encoaraging the zealous, lashing on the sluggish. His soldiers desisted only hours later. Weary of slaughter, they took the remaining 3000 men prisoners. On a narrow area 48,000 corpses lay in heaps. Both Aemilius Paulus and Servilius had fallen, Varro had escaped witha few cavalrymen, a few of the heavily armed and the greater part of the lightly armed men. Thousands fell into the hands of the victors in the village of Cannae and in both camps. The conquerors had lost about 6,000 men. These were mostly Iberians and Gauls.
A battle of complete extermination had been fought, most wonderfully through the fact that in spite of all theories, it had been won by a numerical inferiority. Clausewitz said "concentric action against the enemy behooves not the weaker" and Napoleon taught "the weaker must not turn both wings simultaneously." The weaker Hannibal had, however, acted concentrically, though in an unseemly way, and turned not only both wings, but even the rear of the enemy.
Arms and the mode of combat have undergone a complete change during these 2000 years. No attack takes place at close quarters with short swords, but firing is used at thousands of meters range; the bow has been replaced by the recoil gun, the slingshot by machine guns. Capitulations have taken the place of slaughter. Still the greater conditions of warfare have remained unchanged. The battle of extermination may be fought today according to the same plan as elaborated by Hannibal in long forgotten times. The hostile front is not the aim of the principal attack. It is not against that point that the troops should be massed and the reserves disposed; the essential thing is to crush the flanks. The wings ought not to be sought at the advanced flank points of the front, but along the entire depth and extension of the hostile formation. The extermination is completed by an attack against the rear of the enemy. The cavalry plays here the principal role. It need not attack "intact infantry," but may wreak havoc among the hostile masses by long range fire.
A condition of success lies, it is true, in a deep formation of the hostile forces with shortened front through massing of reserves, thus deepening the flanks and increasing the number of combatants forced to remain in inactivity.
The Battle of Cannae
Alfred, Graf von Schlieffen