Soon thereafter on a raw rainy day a general engagement came on, unlocked for by the Romans.

From the earliest hour of the morning the Roman light troops had been skirmishing with the light cavalry of the enemy; the latter slowly retreated, and the Romans eagerly pursued it through the deeply swollen Trebia to follow up the advantage which they had gained.
Suddenly the cavalry halted; the Roman vanguard found itself face to face with the army of Hannibal drawn up for battle on a field chosen by himself; it was lost, unless the main body should cross the stream with all speed to its support.

Hungry, weary, and wet, the Romans came on and hastened to form in order of battle, the cavalry, as usual, on the wings, the infantry in the centre.

The light troops, who formed the vanguard on both sides, began the combat: but the Romans had already almost exhausted their missiles against the cavalry, and immediately gave way.

In like manner the cavalry gave way on the wings, hard pressed by the elephants in front, and outflanked right and left by the far more numerous Carthaginian horse.
But the Roman infantry proved itself worthy of its name: at the beginning of the battle it fought with very decided superiority against the infantry of the enemy, and even when the repulse of the Roman horse allowed the enemy's cavalry and light-armed troops to turn their attacks against the Roman infantry, the latter, although ceasing to advance, obstinately maintained its ground.