​TREBIA (mod. Trebbia), a river of Cisalpine Gaul, a tributary of the Padus (Po) into which it falls some 4 m. west of Placentia (Piacenza). It is remarkable for the victory gained on its banks by Hannibal over the Romans in 218 B.C. The latest 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 W. J. Kromayer in Anzeiger der phil. hist. Klasse der k. Akademie der Wissenschaften, Vienna, October 14, 1908). Its valley is followed past Bobbio by the modern highroad from Piacenza to Genoa (88 m.).