Hannibal
The Senate had ordered Sempronius Longus to bring his army from Sicily to meet Scipio and to face Hannibal.  Hannibal was in position to head him off. Sempronius avoided Hannibal and joined Scipio near the Trebbia River near Placentia. At Trebia, Hannibal defeated the Roman infantry by a surprise attack from an ambush on the flank.