Things become even more obvious when we notice that Aemilius Paullus, who was seriously wounded, had an opportunity to escape, but declined the offer of a horse to flee, even though the battle had become hopeless. He was like a captain who decides to go down with his sinking ship, or like a Japanese general committing seppuku when defeated. If Varro had been in command, there would have been no reason for Aemilius Paullus to willingly die, but being in command he would have felt guilty and disgraced for having led his splendid army to its doom. Varro escaped, and he was accepted with open arms in Rome and thanked for not despairing of the Republic.
Not only that, but later he was given further military command. There is no way that this would have happened had he been regarded as responsible for the disaster, had it been the outcome of his decision. It seems likely that the reason why Polybius attributed the command to Varro on the day of battle, was to protect the reputation of his employer, the Aemilian family, to which Aemilius Paullus belonged. Varro became a convenient scapegoat.
A different point is the size of the Roman horse. According to Polybius, the Roman cavalry consisted of “over” 6,000. He doesn’t say approximately 6,000, he doesn’t say about 6,000, he says over 6,000. But he doesn’t say how much over, so this is a very open thing. He also says that in situations of emergency, the Romans increased the number of horse accompanying each legion. Instead of having 200 or 250, it became 300 or more. By the time of Polybius the standard cavalry contingent was 300 per legion. In case of an emergency this would be increased to 400. So how many horsemen accompanied each legion at Cannae? It seems very likely that they were probably between 300 and 400, not 150 or 200.
If you assume 400, then the Romans had 3,200 Equites, and since the Italian allied cavalry was required to be three times as numerous, the Italians would have numbered over 9,000. So you have 3,200 plus 9,000, together over 12,000. Hannibal had only 10,000. Why, then, make reference to “over 6,000” when alluding to the Roman horse? The reason is easy to divine. The Romans, who regarded themselves as the best warriors of their time, needed some excuse for their catastrophic defeat by an enemy they outnumbered practically 2 to 1 in infantry. The excuse became “well, we had superior numbers in infantry, but Hannibal enjoyed a vast advantage in cavalry, and that is why he was able to beat us.” The official account claims that the Carthaginians had 10,000 horse and the Romans only 6,000 (or, ahem, “over” 6,000). But if you read carefully, it becomes clear that the Romans didn’t have 6,000, but a lot more than 6,000, actually, in all likelihood, more than 12,000! At Cannae they had superiority in both infantry and cavalry, and yet Hannibal managed to not only defeat them, but to annihilate their numerically superior forces. The injury to Roman pride and hubris required the doctoring up of the historical record.
Another interesting point is that normally a deployed Roman army looked a little bit like a wide, flat rectangle, with the infantry in the center and the cavalry on the sides. But the army that fought at Cannae looked instead like a much narrower and deeper rectangle, with separate cavalry on the sides. Many authors claim that Varro made a strange and erroneous decision. Instead of having the width of a normal deployment, he piled up the units of soldiers one behind the other and made a very deep formation. What possessed him? Why would he do that? Well, I say no, he didn’t make a bizarre decision. The unusual deployment was not Varro’s (or Paullus’s) idea, it was imposed on the Romans by Hannibal himself, who, like a chess grandmaster, created a situation of Zugzwang for his enemies.
Hannibal knew that the Roman army was vastly superior in number, and that if they deployed in their usual way their front would by far exceed that of the Carthaginian army, which then could have been easily enveloped and destroyed. For that reason, he deployed his army first, in such a manner as to force the Romans to deploy facing him in the limited confine between the Cannae hill and the Aufidus River. Within that limited space, there is no way the Romans could have deployed in their normal way. They were forced to deploy with a narrower front and a deeper formation to have enough room for the cavalry on the sides, between the Roman right flank and the Aufidus River, and between their left flank and the hill of the Cannae citadel. So, it was Hannibal who forced Varro, or rather Aemilius Paullus, who most likely was in command, to employ a much deeper formation than normal. This was all part of Hannibal’s master plan to neutralize the numerical superiority of the Roman infantry.
If the Romans did have 12,000 cavalry, 9,000 Allied and approximately 3,000 Equites, as was most likely the case, and they had divided their cavalry in two equal halves, 6,000 riders on each flank, Hannibal would have had a harder time achieving victory. It would have been a very difficult battle. But Hannibal knew that the Romans would not do that, because the Roman Equites would not ride next to the lesser Italian allies. See, they were noblemen. The Italian allies were second-class. So Hannibal knew the Romans would put their nobility on one side, only 3,000, and the much greater force of 9,000, on the other side. Since the right side was the place of honor, it was easy to predict that the Equites would ride on the right, next to the river. Knowing that, he placed his 6,000-strong heavy Iberian and Gallic horse on his left, facing the Italian nobles.
So, even though the Roman horse had overall numerical superiority, on their right wing the Romans were outnumbered 2 to 1. On the opposite side, between the infantry and the hill, the Allied horse was numerically superior, with 9,000 riders, but Hannibal had the Numidians facing them. He had only 4,000 Numidian riders but they could maneuver in ways the Allied horse could not match. The specialty of the Numidians, the most talented horsemen of Antiquity, was the hit and run attack, to approach, attack, retreat, spin around, move in circles, and keep the enemy in a state of disarray and confusion. And they did. They kept the Allied horse busy and unable to charge or advance in a coordinated manner. Meanwhile, the heavy Celtic and Spanish cavalry on Hannibal’s left wing attacked and destroy the 3,000 Equites. With 6,000 against 3,000 it was hardly a contest, although the fighting became so fierce riders even dismounted to finish off their adversaries on foot. In short time, the Roman cavalry on their right flank was no more, while the Allies on the left struggled in confusion faced with the lethal dance of the Numidians.