As Hannibal’s centre collapsed and the legionaries chased sword in hand after the fleeing warriors, the Roman plan appeared to be working. Varro was still on the left flank, his allied horsemen engaged in sporadic and indecisive skirmishing with the elusive Numidian cavalry. The lack of movement on this wing was entirely satisfactory from the consul's point of view, for his task was simply to protect the flanks of the heavy infantry and allow them to win the great victory. It is questionable to what extent Varro could have observed the progress of the fighting in the centre for the dust would only have added to the confusion, and most unlikely that he knew of the flight of the Roman cavalry on the right, but he may well have been able to see that the Roman foot were steadily pressing forward. Paullus, Servilius and the other officers with the infantry knew that the plan was working and redoubled their efforts to pour more of their reserves into the gap, giving the enemy no opportunity to rally. In the centre of the battlefield a great mass of Roman infantrymen some tens of thousands strong pressed forward to complete the rout of the enemy foot.

The Roman legion was supposed to operate with wide gaps between its maniples and significant intervals between each of the three lines. The openness of its formation allowed the legion to advance without falling into disorder even over comparatively rough terrain. It is impossible, even for well drilled troops, to march in a perfectly straight line, and the more uneven the terrain, the more probable that a unit will veer to one side or the other. The wide intervals between the maniples of the legion allowed them to cope with such deviation without units colliding and merging together and ceasing to be independent tactical entities. The unusual formation adopted by the Roman infantry at Cannae sacrificed this openness and with it most of the flexibility of the manipular system. Varro deployed the maniples on a very narrow frontage but in great depth and reduced the width of the gaps between each unit. As soon as the Roman line began to move forward, these intervals would have tended to disappear, the maniples merging together so that instead of many semi-independent tactical units there was simply one mass. The greater depth of each maniple may have reduced the space normally maintained between each of the three lines, but, even if it had not, the confined nature of the plain between the Aufidius and the hills around Cannae probably had the same result. There may well have been some blurring between the lines as well as amongst the individual maniples as the Roman centre lumbered forward.

Massed battles presented many problems to sculptors and artists. This scene from the early second century AD shows a battle from the Emperor Trajan’s Dacian Wars. The uniforms of the Roman soldiers at Cannae were very different.

The Roman plan relied entirely upon their numerically superior infantry to win the day. The cavalry were there primarily to hold off Hannibal's better quality and more numerous horsemen for long enough to allow the Roman foot to break the Punic line. It was therefore important for the legions to win as quickly as possible. The Roman officers who clustered just behind the fighting line knew this, and were all the more willing to feed men from the reserve lines into the combat. Hannibal had deliberately placed his centre much closer to the enemy than his flanks and, as he hoped, it was there that the infantry fighting first developed.

As the Romans began to make headway against this advanced position, more and more men from the reserve lines were sent to reinforce their fighting line. It was not just the maniples directly behind the engaged units which were drawn into the struggle, for Polybius tells us that large numbers of men on either side were sucked into the combat. The distinction between separate maniples had already started to dissolve as the army advanced, and especially amongst the units who then came into contact with the enemy, but rapidly vanished altogether as more and more men were packed into the combat being fought on a very small front. The Romans' breakthrough was achieved at the price of their good order and their infantry were now more like a crowd than an organized body divided into distinct sub-units closely controlled by their officers.42

Yet victory must have seemed close as the Roman mass punched straight through the very centre of Hannibal's line. The Celts and Spaniards on either side of the gaping hole in their line seem not to have broken, but retreated in better order. The concentration of the overwhelming weight of the Roman attack on the centre and the use of most reserves there probably meant that they were under far less pressure. The Romans surged forward until they were level with the starting position of the flanks of Hannibal's main line and kept going, for there was nothing to oppose them. The attack still had considerable momentum, but very little order, and was no longer under anyone's control. In such a mass an officer of any rank could only influence the men immediately around him.

The fourth phase of the battle. As the Romans pour more and more reserves into the fighting, the pressure of their numbers breaks through the Punic line. The Gallic and Spanish warbands in the centre break and stream to the rear. Scenting victory, the Romans pursue. By now their units have lost much of their order and the three neat lines in which they began the battle are only a memory. In the meantime Hasdrubal leads his cavalry against the rear of Varro's Italians. The latter panic and flee before a blow has been struck.

As the Roman mass streamed forward, they found themselves with Hannibal's Libyan infantry arrayed in columns on either side of them. The Libyans had as yet played no part in the fighting and were fresh and in good order. There were probably no more than 8,000-10,000 of them, divided into two forces each roughly the size of a Roman legion. The similarity went further, since they were now dressed and armed with Roman equipment stripped from the dead of Trebia and Trasimene, although it is unlikely that they had adopted manipular organization and tactics and probable that they still fought as a phalanx. We do not know who gave the orders - perhaps Hannibal had ridden from his routed centre and gone in person to one of the Libyan columns, sent a messenger, or simply explained in detail to the Libyans’ commanders before the battle what was required of them - but the column on the left turned to form a line facing to the right and those on the right turned to face left. Then the two phalanxes marched forward and attacked into the flanks of the crowded mass of pursuing Roman soldiers.43

The Romans were in no position to form fighting lines to face either of the new threats. The maniples were hopelessly intermingled and beyond the control of their leaders. The already confused situation was probably exacerbated by the Roman-like appearance of the bodies of infantry moving towards them and it may have taken some time to realize that these were hostile. The loss of a clear sense of direction seems to be common under the stress of combat and few men may have realized that there should not be any friendly troops approaching from that direction. Small groups of soldiers may have turned to form rough lines facing the enemy, but they lacked missile weapons, were fatigued through combat and never formed a coherent line. The Libyan's charge stopped the Roman advance dead, robbing it of all momentum. There were now no organized reserves in the Roman army to feed into the combat and renew the surge forward. Officers improvised as best they could, but movement in the packed ranks was probably difficult and became even harder as it contracted under the pressure of the twin enemy attacks. The two phalanxes of Libyans gripped the Romans like a vice, and around them the surviving Celtic and Spanish troops, joined perhaps after a while by some of the routers, pressed round to add to the fighting line. The Romans were now fighting on three sides, but unable to support the combat in any direction properly.

In the meantime events had occurred elsewhere on the battlefield which would seal the fate of the Roman centre. Hasdrubal had led his close order cavalry in a devastatingly brutal charge against the Roman right wing, shattering and virtually destroying it in a brief pursuit. The Carthaginian had kept his men under tight control and, when they had rested and reformed, he led them behind the Roman main line, moving against Varro on the left, and ignoring the massed infantry in the enemy centre. Varro's allied horsemen were still engaged in their stand-off with the Numidians, but the sight of the lines of Hasdrubal's Gauls and Spaniards approaching from the rear utterly shattered their spirit. Without waiting for the Carthaginians to charge home, the Roman left wing dissolved into a panicked flight in which the consul joined. Their position was untenable, and, if they had in fact formed with their flank on the hills around Cannae, any delay in flight might have resulted in their being trapped. They could not have won any combat with a more numerous enemy attacking from two sides, but their flight sealed the fate of the Roman army. Hasdrubal had once again kept his men closely in hand, helped perhaps by the enemy's swift flight, which meant that the Punic horse did not have to go through with their charge and fight a melee. He gave orders for the Numidians to pursue the fleeing enemy horsemen - a task to which they were ideally suited - and led his own command against the rear of the Roman foot. The confused mass of 50,000 or so Roman and allied heavy infantryman and maybe as many as 20,000 velites was now surrounded.44

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