TREBIA (mod. Trebbia), a river of Cisalpine Gaul, a tribu tary of the Padus (Po), into which it falls some 4m. west of Placentia (Piacenza). Its valley is followed past Bobbio by the modern high road from Piacenza to Genoa (88m.). It is remark able for the victory gained on its banks by Hannibal over the Romans in 218 B.C. Kromayer's investigations make it clear that Polybius's account, according to which the battle took place on the left bank of the river, is to be preferred to that of Livy (see Antike Schlachtfelder, iii. I [Weidmann, 1914], 47 sqq.). T. Frank in Journal of Roman Studies, ix. 202 (1919), suggests that the Placentia referred to in both accounts lay west of the river, i.e., some 15 miles west of the later Placentia, refounded in 19o.
Scipio (the father of Scipio Africanus) had advanced to meet Hannibal two days' march west of Pavia, but was defeated in a cavalry engagement, and in a forced night march recrossed the Ticinus and the Padus (probably at Placentia itself) and then took up a new position near Clastidium on the south bank of the latter river, about 20M. west of Placentia : while Hannibal, finding himself unable to cross the Ticinus, marched westward up the Padus until he could cross it, and so came up with the Romans and offered battle, which was, however, not accepted ; and he therefore encamped five miles away from them. Scipio, however, in con sequence of the desertion of over 2,000 of his Gaulish auxiliaries, retreated to the further bank of the Trebia, where he was joined by Sempronius, the other consul, who had brought his army from Ariminum. Hannibal followed them and encamped once more about 4m. from them : and as a decisive victory was imperative, in order that he might win over the Gauls to his side, and he had about 40,000 men, or as many as the two armies together, he did nothing to prevent their junction. A successful cavalry skirmish
with the Carthaginian foragers encouraged Sempronius to over come his colleague's objections to a general engagement, and the Romans marched across the Trebia. The battlefield was almost flat, but on the south was rising ground traversed by a number of fairly deep, broad stream-beds, in one of which Hannibal con cealed a force of 2,000 men under his young brother Mago, which was to have a decisive effect on the fortunes of the day. At the same time he sent his 8,000 light troops out to cover the advance of his main body of infantry, consisting of 20,000 Iberians, Celts and Libyans, while his cavalry (over io,000, including his Celtic allies) were placed on the wings, with the elephants in advance of them. The Roman cavalry was soon driven back, and the Car thaginian light troops, Numidian cavalry and elephants attacked the Roman infantry on the flanks, so that the wings were put to flight and driven into the river, while Mago's force suddenly fell on the rear of the Roman centre and worked destruction on the rear ranks. The front ranks of the Roman centre, on the other hand, to a number of some i o,000 men broke through the Car thaginian centre with great slaughter, but seeing that the wings had been completely driven back, they took no further part in the battle, but marched in good order into Placentia, where they were joined by the cavalry. The greater part of the rest of the army, entirely surrounded by superior numbers, met its death in the river itself. The battle has been rightly described as a Cannae on a small scale, and the similarities are certainly striking, as will be seen by a comparison of the two accounts. (T. A.)