The second Punic War began in 218, and immediately after the onset of hostilities, Hannibal made his way across the Alps to the region the Romans called Cisalpine Gaul. His plan was to march down the Italian peninsula, defeat the Roman Army, turn the non-Roman Italians to his side, and force the Romans to surrender. Strategically innovative, a quick thinker tactically, and courageous in battle, Hannibal had all the attributes of a military genius. Even so, his personal ambition was placed above the goals of the Punic War Council, and he assumed that continuing victories would maintain support for him back home.
Hannibal had an army of forty to fifty thousand men so the Romans knew it would take a large coordinated force to defeat him. Unfortunately, they could not assemble one because they were handicapped by their command structure and the arrogance of their leaders. As we have discussed before, the Republic had two senior magistrates (consuls) who were elected simultaneously for one year. In time of war, each consul served as commander of one of the two armies. Jealousy and personal ambition often put the two commanders on different paths and divided the army - often with disasterous results as the story of the battle of Trebia will show us.
In this case, the two consuls prepared to face Hannibal were Publius Cornelius Scipio (father of Africanus) and Tiberius Sempronius Longus. Scipio had been severely injured in a skirmish with Hannibal a week or two before Trebia. His losses were heavy and Scipio was humbled by his first encounter with the Carthaginian general. Longus, however, was newly arrived and anxious to fight. He immediately sent out a reconnaissance force which surprised and defeated a Carthaginian raiding party. This small victory fooled Longus into thinking he could defeat Hannibal in a larger battle.
Longus argued with Scipio about what they should do. Scipio was cautious and advised against a major battle before the end of the year (it was late December). Longus sought the glory of victory and the rare chance to command two armies. He knew if Rome waited until the next year to fight Hannibal, consular elections would intervene and the glory might go to the next commander instead of himself. Longus foolishness would set the stage for disaster.
Hannibal knew, from spies, the new Roman commander was anxious for battle so he set a trap. He located his camp on the west side of the Trebia River and then found a place to hide 2000 hand-picked men south of the camp. Early, on the day of the battle, Hannibal sent his cavalry to the Roman camp. Their purpose was to feign an attack and induce the Romans to fight. Longus took the bait, brought is army out, and followed the Carthaginians. When Longus reached the east side of the river he could see that the Carthaginians were on the opposite side so he sent his men across. The water was chest deep and freezing. The Roman army had not eaten breakfast so they were exhausted by the time they reached the other side. Meanwhile Hannibal’s men ate at their leisure and warmed themselves in front of campfires.
The battle began in a snowstorm with the Romans pushing into the middle of the Carthaginian line. Once the Romans were deep in the center, the Punic cavalry closed the wings against an inferior Roman cavalry and began to destroy the Roman infantry. Then, when the moment was right, the hidden Carthaginian force attacked from behind the Roman position. After heavy losses, Longus decided to retreat, so he took the majority of his force northward to a bridge, crossed the Trebia, and proceeded to the town of Placentia. Many of his men tried to cross the river to return to the Roman camp but were cut down by the Carthaginians.
The Romans had lost 15,000 men, a number that would have been greater if the snowstorm hadn’t prevented a stronger pursuit. Defeat meant Longus’ career was over, but the Romans didn’t learn their lesson. They would have to be crushed at Lake Trasimene and Cannae, before adopting a war of attrition against a man they could not defeat in a face to face battle. Later, in 202 BC, the younger Scipio was able to defeat Hannibal using tactics the Romans had learned from him.