With the outbreak of the Second Punic War, Carthaginian forces under Hannibal successfully moved against the Roman city of Saguntum in Iberia. Completing this campaign, he began planning to cross the Alps to invade northern Italy. Moving forward in the spring of 218 BC, Hannibal was able to sweep aside those native tribes that attempted to block his path and entered the mountains. Battling harsh weather and rough terrain, Carthaginian forces succeeded in crossing the Alps, but lost a significant part of this numbers in the process.
Surprising the Romans by appearing in the Po Valley, Hannibal was able to earn the support of rebelling Gallic tribes in the area. Moving quickly, Roman consul Publius Cornelius Scipio attempted to block Hannibal at Ticinus in November 218 BC. Defeated and wounded in the action, Scipio was forced to fall back to Placentia and cede the plain of Lombardy to the Carthaginians. Though Hannibal's victory was minor, it had significant political repercussions as it led to additional Gauls and Ligurians joining his forces which raised his army's numbers to around 40,000.
Concerned by Scipio's defeat, the Romans ordered Consul Tiberius Sempronius Longus to reinforce the position at Placentia. Alerted to Sempronius' approach, Hannibal sought to destroy the second Roman army before it could unite with Scipio, but was unable to do so as his supply situation dictated that he assault Clastidium. Reaching Scipio's camp near the banks of the Trebia River, Sempronius assumed command of the combined force. A rash and impetuous leader, Sempronius began making plans to engage Hannibal in open battle before the more senior Scipio recovered and resumed command.
Aware of the personality differences between the two Roman commanders, Hannibal sought to fight Sempronius rather the wilier Scipio. Establishing a camp across the Trebia from the Romans, Hannibal detached 2,000 men, led by his brother Mago, under the cover of darkness on December 17/18. Sending them to the south, they concealed themselves in streambeds and swamps on the flanks of the two armies. The following morning, Hannibal ordered elements of his cavalry to cross the Trebia and harass the Romans. Once engaged they were to retreat and lure the Romans to a point where Mago's men could launch an ambush.
Ordering his own cavalry to attack the approaching Carthaginian horsemen, Sempronius raised his entire army and sent it forward against Hannibal's camp. Seeing this, Hannibal quickly formed his army with infantry in the center and cavalry and war elephants on the flanks. Sempronius approached in the standard Roman formation with three lines of infantry in the center and cavalry on the flanks. In addition, velite skirmishers were deployed forward. As the two armies collided, the velites were thrown back and the heavy infantry engaged.
On the flanks, the Carthaginian cavalry, making use of their greater numbers, slowly pushed back their Roman counterparts. As pressure on the Roman cavalry grew, the flanks of the infantry became unprotected and open to attack. Sending forward his war elephants against the Roman left, Hannibal next ordered his cavalry to attack the exposed flanks of the Roman infantry. With the Roman lines wavering, Mago's men sprang from their concealed position and attacked Sempronius' rear. Nearly surrounded, the Roman army collapsed and began fleeing back across the river.
As the Roman army broke, thousands were cut down or trampled as they attempted to escape to safety. Only the center of Sempronius' infantry, which had fought well, was able to retire to Placentia in good order. As with many battles in this period, precise casualties are not known. Sources indicate that Carthaginian losses were light, while the Romans may have suffered up to 20,000 killed, wounded, and captured. The victory at Trebia was Hannibal's first great triumph in Italy and would be followed by others at Lake Transimene (217 BC) and Cannae (216 BC). Despite these stunning victories, Hannibal was never able to completely defeat Rome, and was ultimately recalled to Carthage to aid in protecting the city from a Roman army. In the resulting battle at Zama (202 BC), he was beaten and Carthage was forced to make peace.
