The newly elected Roman Consuls, Gaius Terentius Varro and Lucius Aemilius Paullus, who had both run on a platform of taking the war to Hannibal, were anxious to begin their tenure with military achievement. Counter to the delaying tactics of the Dictator Fabius Maximus, Varro and Paulus immediately formed a large force to deal with the Carthaginians ravaging southern Italy. While ancient sources offer conflicting reports, it can be safe to assume that between the two, Consuls, they levied a force of nearly 80,000 men.
Hannibal meanwhile, still attempting to subvert Roman authority in the allied areas of Italy, was waiting for the Roman with approximately 40,000 men; Gauls, Carthaginians and Numidian cavalry. Despite the popular conception that the elephants played a major role in the campaign, by this time, all of his elephants had died. Hannibal, despite his numerical inferiority had such an overwhelming strategic edge, that he was eager to meet the new Roman challenge. Theoretically, the Roman tactic of crushing Hannibal between two large armies should have spelled his doom, but Hannibal's brilliance allowed him to turn the tables once the engagement got under way.
On August 2, 216 BC, in the Apulian plain, near Cannae and near the mouth of the Aufidus River, the 2 great armies came face to face. The Consul Varro was in command on the first day for the Romans, as the consuls alternated commands as they marched. Paullus, it has been suggested, was opposed to the engagement as it was taking shape, but regardless still brought his force to bear. The two armies positioned their lines and soon advanced against one another.
The cavalry was to meet first on the flanks. Hasdrubal, commanding the Numidians, quickly overpowered the inferior Romans on the right flank and routed them. Pushing them into the river and scattering any opposing infantry in his path, Hasdrubal dominated the right flank and was quickly able to get in the rear of the enemy lines. While the much superior Numidians dealt quickly with their Roman counterparts, such was not the case with the infantry.
As Hasdrubal was routing the Roman horse, the mass of infantry on both sides advanced towards each other in the middle of the field. The Iberian and Gallic Celts on the Carthaginian side, while fierce, were no match for Roman armament and close-quartered combat. Initially, the vast numerical advantage of the Legions pushed deep into the middle of the Carthaginians. While the Celts were pushed back, they didn't break, however. They held as firm as they could, while Hasdrubal's cavalry pushed around to the rear of the enemy and the Carthaginian infantry held firm on the immediate flanks. The Romans soon found that their success in the middle was pushing them into a potential disaster. As they victoriously fought farther into the center of Hannibal's lines, they were actually walking themselves right into being completely encircled.
Just as the Romans were on the brink of crushing the enemy center, the Carthaginian flanks were brought to bear and the pressure pinned in the Roman advance. Hasdrubals' cavalry completed the circle by forcing the rear of the Roman line to turn back and form a square. All around, the massive bulk of the Roman army was forced into confined space. Hannibal brought his archers and slingers to bear and the result in the confines was devastating. Unable to continue the original break through against the Celts in the center of Hannibal's lines, the Romans were easy prey for the Carthaginians. Hannibal, with complete fury, encouraged his own men, under fear of the lash, if they weren't zealous enough in the slaughter.
In the midst of the battle the Consul, Paullus, was wounded (either early or late depending on Livy or Polybius as the source). He valiantly attempted to maintain the Roman ranks, though vainly. While the commander of the day, Varro, fled the battle, Paullus stayed the course trying to save his army. In the end, it was a terrible slaughter and Paullus would be dead with the bulk of his men. Romans trying to escape were hamstrung as they ran, so the Carthaginians could concentrate on those who were still fighting, but allow time to return and kill the crippled later. In a fast and furious display of death, Hannibal ordered his men to stand down only a few short hours after they originally encircled the enemy.
On a small strip of land where the Romans were bottled up, estimates as high as 60,000 corpses were piled one on top of another. Another 3,000 Romans were captured and more staggered into villages surrounding the battlefield. Hannibal, however, still trying to win the hearts of the Italian Roman allies, once again released the prisoners, much to the dismay of his commanders. In salute to the fallen Paullus, Hannibal also honored him with ceremonial rituals in recognition of his valiant actions.
In the end, perhaps only as many as 15,000 Romans managed to escape with Varro. These survivors were later reconstituted as two units and assigned to Sicily for the remainder of the war as punishment for their loss. Along with Paullus, both of the Quaestors were killed, as well as 29 out of 48 military tribunes and an additional 80 other senators (at a time when the Roman Senate was no more than 300 men). The rings signifying membership in the Senate and from those of Equestrian (Knight class or the elite class after Patrician) status were collected from the dead in baskets and later thrown onto the floor of the Carthaginian Senate in disrespect. In contrast, Hannibal's losses numbered only between 6,000 and 7,000 men, of whom, these were mostly his Celtic recruits. Once again Hannibal proved brilliant in battlefield strategy, using the enemy's tactics against itself and routing an army twice the size of his own.
In less than a year since the disaster at Trasimenus, the Roman's greatest loss was in history put the state into a panic. There was nothing keeping Hannibal from sacking Rome itself at this point, other than Hannibal. His generals again urged him to not waste any more effort and go for the final kill, but Hannibal was reluctant. Still believing he couldn't take Rome itself, he preferred his strategy of pursuing revolt among the Roman allies.
Despite this tremendous loss, the following defection of many allied cities, and the declaration of war by Philip of Macedon that was soon to come, the Romans showed a resiliency that defined them as people. According to Livy, "No other nation in the world could have suffered so tremendous a series of disasters and not been overwhelmed." The truth of that nature was self evident. While some in the Senate, such as Lucius Caecilius Metellus were ready to abandon the Republic as a lost cause, others like Scipio propped up the flagging Roman spirit with encouragement and undying oaths of loyalty to Rome.
Shortly after Cannae, the Romans rallied back, declaring full mobilization. Another dictator, M. Junius Pera, was elected to stabilize the Republic. New legions were raised with conscripts from previous untouched citizen classes. As the land owning population was heavily diminished by losses to Hannibal, the Romans took advantage of the masses. Those in debt were released from their obligations, non-land owners were recruited and even slaves were freed to join the legions. In so doing, the Romans also refused to pay ransoms to Hannibal for any captured legionaries who still remained. Hannibal, it was suggested, lost his spirit, understanding that Rome would rather sacrifice its own than surrender anything to him. While fortune would still be with Hannibal for some time, the war of attrition would only benefit Rome.