While Hannibal's march through Gaul was relatively uncontested, the survival of his army through the Alps, let alone his subsequent victories was a marvelous achievement. Malnourished, weather-beaten and exhausted, the Carthaginian force was met with resistance by many of the local Gallic tribes.
The Allobroges offered the first challenge by attacking the rear of his column. Other Celts harassed Hannibal's baggage trains, rolling large boulders from the heights onto the Carthaginian columns, causing panic and death among the victims. Fierce resistance throughout the march debilitated Hannibal's army. The cold altitudes of the Alps certainly were no benefit to some under-dressed tribal warriors in his forces.
Cold and hungry, Hannibal and his army stormed a Gallic town on the 3rd day of the mountain hike. The resulting plunder offered some relief in the form of food and supplies, but constant pressure from the Celts, landslides, continuing bad weather and poor supply made the success of the operation all the more memorable. By the 15th day, Hannibal stepped down into the foothills of northern Italy. With only 20,000 infantry, 6,000 cavalry and only a few remaining elephants, his army was decimated by the journey. Fortunately for Hannibal, the Celts on the Italian side of the Alps were far friendlier and Gallic recruits pushed the Carthaginians back up to between 30,000 and 40,000 men.
Meanwhile, the Romans were waiting in Cisalpine Gaul under Scipio the Elder. With a small force already positioned to keep the Gauls in check, Scipio moved to intercept Hannibal. At the Battle of Ticinus, in late 218 BC, the 2 forces were first engaged in a small confrontation. Light troops send by Scipio to scout the enemy were met by Numidian cavalry and soundly defeated. As only a prelude of things to come, the most significant result was the wounding of Scipio and the opening of additional Gallic recruitment to Hannibal. The Romans were forced to withdraw to Placentia, under Manlius, to plan for another attack.
With the minor victory at Ticinus, but more importantly the withdrawal of the Romans, Gallic and Ligurian recruits were now eager to join against Rome. Hannibal's army, significantly supplemented, was now ready to push full force into Italy. At the Trebbia River, the Romans combined the forces of Scipio's remaining legions with those of Ti. Sempronius Longus. In December of 218 BC, with Scipio out of commission from his wounds, the eager Sempronius threw caution to the wind and advanced against Hannibal.
The Battle of the Trebbia River was the first significant engagement of the war and the first real test for Hannibal and his army. He brilliantly anticipated Sempronius' impetuousness and set up an ambush. Before the impending fight, Hannibal sent a force of 2,000, 1,000 each of infantry and cavalry, under the command of his brother Mago to conceal themselves in the riverbeds. When dawn broke, Numidian cavalry harassed the Roman camp, angering Sempronius and stirring him to action. The main Roman army approached the Trebbia, pushing the Numidians back across, completely unaware of the trap set for them.
Hannibal waited with his army arranged as a screen with 10,000 cavalry and elephants, flanking the infantry of 30,000. Sempronius faced him with upwards of 40,000 men. Roman light infantry (velites) met the enemy first and were badly beaten, though they would be largely responsible for eliminating the remainder of Hannibal's few elephants. Numidian cavalry crushed the Roman cavalry on the flanks and things were bad for Sempronius from the start.
When the main armies met, the situation stabilized slightly for Rome, but the flanking pressure from the superior Numidian cavalry soon began to turn the tide. At the critical juncture, Mago's ambush was sprung, and the Romans were finished. Demoralized by the bitter cold of December in northern Italy, the Romans were routed, cut down as they fled. In the end, nearly half of Sempronius' force was lost, about 15 to 20,000 men. The remainder of the Roman army managed to escape to Placentia.
Hannibal's losses were far less. His elephants were gone, but of his regular army only the newly recruited Gauls suffered at all. At Trebbia, Hannibal proved his superior leadership in understanding the psychology of his opponent, his tactical strategy and in propaganda warfare. With his victory, Hannibal released the bulk of any prisoners captured with the intention of securing favor among Rome's allies throughout Italy. While theoretically an excellent concept, it was this sort of continuing hope for open rebellion that played a major factor in his eventual undoing.