It is clear from this that Hannibal was expecting an immediate battle, and our sources tell us that Longus was also ready to fight, while only Scipio advised against battle, suggesting that to delay would weaken Hannibal and strength the Romans, although this might be an later invention. As a strategy it may well have made sense. Hannibal had an experienced army, while the Roman army at this time was raised on an annual basis, and even if it included experienced soldiers, needed time to become a good fighting force. However, very few Roman commanders of the period would have refused battle against what was a smaller army.
Although at first glance it was the Romans who advanced into battle, it was Hannibal who decided when and where the battle would be fought. On an initial inspection, the plain west of the Trebia was an ideal Roman battleground, free of major obstacles. However, during his scouting, Hannibal found a gully, where he placed 2,000 men commanded by his brother Mago. The next morning, Hannibal sent his Numidian cavalry across the river, with orders to provoke the Romans into battle with a series of attacks on their outposts. Longus reacted exactly as Hannibal hoped, sending his own cavalry and skirmishers against the Numidians, before ordering his main army to prepare to cross the river and offer battle. This was a slow process, especially for such a large and inexperienced army, and probably took several hours, before the Roman army, tired and wet, was in place facing Hannibal's smaller but rested force.
The two forces were now drawn up for the first major set piece battle of the war. Hannibal formed his 20,000 heavy infantry into a thin line in the middle of his line, with the Gauls in the middle and the Libyans and Spanish on their flanks, with his 10,000 cavalry split evenly between the two wings, and of course Mago's 2,000 men hidden to the Roman rear. The Roman army was formed up in their standard formation, with the 16,000 legionares in the centre, 10,000 allied infantry on each flank, and finally 2,000 cavalry on each flank.
Longus ordered his army to advance in the traditional Roman manner, slowly and in good order. The first clash was between the skirmishers of the two armies. The Roman troops were outnumbered, probably tired, and of a lower quality than Hannibal's own skirmishers, and were quickly despatched. The Roman cavalry suffered in a similar way, and soon Hannibal's cavalry and skirmishers were attacking the flanks of the Roman army. Only in the centre did the Romans do better, where they outnumbered Hannibal's troops. Even Mago's attack, which did much to finish off the Roman allies, did not defeat the legions, who broke through Hannibal's centre. Without reserves, Hannibal might have been in trouble, but by now it was clear that the Romans had lost, and the 10,000 legionaries made no attempt to rejoin the battle, instead fleeing back to Placentia.
This was the first defeat Hannibal had inflicted on the Roman legions, once again defeating a consul, although both consuls escaped, as did much of their army. Once again, Hannibal had shown himself to be a superior commander to his Roman opponents, and this time the Legions had also been defeated. However, far worse was to come for Rome.