A key reason for some historians' disparagement of the Roman cavalry were the crushing defeats, at the Trebia and at Cannae, that it suffered at the hands of the Carthaginian general Hannibal during the latter's invasion of Italy (218-6 BC). But Sidnell points out these reverses were not due to poor performance by the Romans, who fought with their customary courage and tenacity, but to the Hannibalic cavalry's far superior numbers and the operational flexibility afforded by his Numidian light cavalry. Hannibal's already powerful cavalry (6,000 men) that he brought over the Alps, consisting of Spanish cavalry and Numidian light, was swollen by the adherence of most of the Gallic tribes of northern Italy, who provided an additional 4,000, bringing his horse up to 20% of his total force. At Cannae, 6,000 Roman horse (including Italian confederates) faced 10,000 Carthaginians, and on the Roman right wing, the Roman cavalry of 2,400 was probably outnumbered by more than 2 to 1 by Hannibal's Spaniards and Gauls. It is on this wing that the Roman disaster at Cannae was determined, as the Roman cavalry were overwhelmed and broken. In the words of Polybius: "As soon as the Spanish and Celtic horse on the (Carthaginian) left wing came into contact with the Roman cavalry... the fighting which developed was truly barbaric... Once the two forces had met they dismounted and fought on foot, man to man. Here the Carthaginians finally prevailed, and although the Romans resisted with desperate courage, most of them were killed..." The fact that the Romans dismounted has been used to support the thesis of a Roman cavalry that lacked confidence in its horsemanship and was in reality just a mounted infantry. But since the Carthaginian cavalry also dismounted, Livy's explanation is more credible, that fighting on horseback was impractical in the confined space between the right flank of the Roman infantry and the river Aufidus.