At this stage a select Carthaginian band of 1000 infantry, and as many horsemen, under the leadership of Mago, Hannibal's youngest brother, suddenly emerged from an ambush in the rear of the Roman army, and fell upon the densely entangled masses.

The wings of the army and the rear ranks of the Roman centre were broken up and scattered by this attack, while the first division, 10,000 men strong, in compact array broke through the Carthaginian line, and made a passage for itself obliquely through the midst of the enemy, inflicting great loss on the opposing infantry and more especially on the Gallic insurgents.

This brave body, pursued but feebly, thus reached Placentia.

The remaining mass was for the most part slaughtered by the elephants and light troops of the enemy in attempting to cross the river: only part of the cavalry and some divisions of infantry were able, by wading through the river, to gain the camp whither the Carthaginians did not follow them, and thus they too reached Placentia.( 1) Few battles confer more honour on the Roman soldier than this on the Trebia, and few at the same time furnish graver impeachment of the general in command; although the candid judge will not forget that a commandership in chief expiring on a definite day was an unmilitary institution, and that figs cannot be reaped from thistles.

The victory came to be costly even to the victors.