Sempronius avoided Hannibal and joined Scipio near the Trebbia River near Placentia . At Trebia , Hannibal deafeated the Roman infantry by a surprise attack from an ambush in the flank . But this gain was not without its loss , as Sempronius avoided Hannibal 's watchfulness , slipped around his flank , and joined his colleague in his camp near the Trebia River near Placentia .