He wanted to crush the Roman army with his superior land forces in their own territory, especially since he counted on the disaffection of Rome's Italian allies. Thus he crossed the Iberus in the spring of 218, and, after bloody battles with Spanish tribes, he marched with about 40, 000 men across the Pyrenees. Once in Gaul, he hastened to the Rhone River without meeting resistance and, within a week, transported his army and war elephants across the river.

Meanwhile, the Roman consul Publius Cornelius Scipio, who had transported his troops by sea to Massilia (Marseilles), was moving north on the right bank of the Rhone, but when he heard that Hannibal had already crossed the river, he sent his brother Gnaeus with two legions to Spain, while he himself returned to northern Italy. Hannibal, on the other hand, wanted to cross the Alps and reach the Po Valley before the Romans were able to collect their forces against an unexpected invader. In 15 days he marched through rugged, unknown mountain passes, with his enormous army of diverse origin and language and his 38 war elephants, in the midst of enemy attacks, landslides, and early autumn snow—a heroic feat which has captured the imagination of historians and poets alike.

When Hannibal finally reached the Po Valley, his army was reduced to half its former size and most of his war elephants were lost. And yet, when he met the army of Publius Scipio at the Ticinus River, Hannibal's Numidian cavalry won a decisive victory over the Romans. Scipio, who was seriously wounded, withdrew to the Trebia River south of Placentia, where the consular army of Titus Sèmpronius Longus, recalled by the Senate from Sicily, joined him. Using the tactics of both ambush and outflanking the enemy, Hannibal defeated the combined armies, causing the loss of about 20, 000 Roman soldiers.

After spending the winter in the Po Valley, where he gained many recruits among the Gauls and Ligurians, Hannibal crossed the Apennines in the spring of 217. By ravaging Etruria he provoked the pursuit of the new consul Gaius Flaminius, whom Hannibal trapped with two legions in a defile on the northern shore of Lake Trasimenus. Rushing down from their ambush on the opposing hills, Hannibal's troops annihilated almost the entire army and, shortly afterward, intercepted and destroyed the cavalry that was sent to aid Flaminius.

Now Hannibal marched to Picenum, where he granted his troops a period of rest in the hope that Rome's Italian allies would defect. He continued to ravage Apulia and Campania without being able to involve the dictator Quintus Fabius, called Cunctator for his tactics of delay, in anything but minor skirmishes. But in the following year, when a new pair of consuls put into effect the aggressive war policy of the Senate, Hannibal beat the Romans in the worst defeat they had ever suffered. This happened at Cannae, where his strategy of outflanking the enemy again brought victory to the Carthaginians over superior numbers.

Now Capua and many other cities in southern Italy revolted against Rome, but Hannibal's weakened forces prevented him from taking full advantage of his victory. Making Capua his headquarters, he changed from an offensive to a defensive policy, mostly because his home government refused to send him adequate reinforcements. Although he was able to capture Tarentum, conquer Bruttium, and win a few minor victories, he gradually lost ground against the superior numbers of the Romans.

Negotiations with Philip V of Macedon and with Hieronymus of Syracuse proved ineffective, and the small band of Numidian cavalry sent to him from Carthage was insufficient for major warfare. In 211, when he was unable to relieve the Roman siege of Capua, Hannibal marched on Rome, pitched camp on the Anio River at a 3-mile distance from the city, but withdrew again to Apulia in the hope that his brother Hasdrubal would bring fresh troops across the Alps from Spain. This hope was shattered in 207, when his brother's bloody head was thrown at his feet as a testimony to the destruction of Hasdrubal's army in the battle of the Metaurus. Hannibal now concentrated his forces in Bruttium, where he held his ground for 4 more years, until he was recalled in 203 to defend Carthage against the victorious army of Publius Cornelius Scipio the Elder (Scipio Africanus Major).

Back in his native land after 16 years of victorious warfare in enemy territory, Hannibal was finally defeated by Scipio Africanus in the battle of Zama. Ironically, Hannibal became the victim of his own strategy: Scipio outflanked and surrounded the Carthaginians with the aid of King Masinissa's Numidian cavalry. Hannibal escaped with only a few horsemen and rushed to Carthage, where he counseled peace. The treaty was concluded in 201.

Elected a suffete (civil magistrate) in 197, Hannibal broke the power of the Carthaginian oligarchy and worked for social and economic reforms. His political enemies accused him in Rome of intriguing with King Antiochus III of Syria. When the Romans sent a commission to investigate the matter, Hannibal fled, first to Antiochus's court at Ephesus, and, after the latter's defeat at Magnesia in 189, to King Prusias of Bithynia.

Hannibal helped his host successfully in a naval battle against King Eumenes of Pergamum, Rome's ally. When another senatorial commission was sent to demand from Prusias the surrender of the famous Carthaginian exile, Hannibal poisoned himself.