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Overland journey to Italy

This campaign was originally planned by Hannibal's brother-in-law Hasdrubal the Fair, who became a Carthaginian general in the Iberian Peninsula in 229 BC. He maintained this post for eight years until 221 BC. Soon the Romans became aware of an alliance between Carthage and the Celts of the Po Valley in Northern Italy. When Hannibal arrived in the Po Valley, roughly 10,000 Celtic tribesmen joined his army.

The Celts were amassing forces to invade farther south in Italy, presumably with Carthaginian backing. Therefore, the Romans pre-emptively invaded the Po region in 225 BC. By 220 BC, the Romans had annexed the area as Cisalpine Gaul. Hasdrubal was assassinated around the same time (221 BC), bringing Hannibal to the fore. It seems that the Romans lulled themselves into a false sense of security, having dealt with the threat of a Gallo-Carthaginian invasion, and perhaps knowing that the original Carthaginian commander had been killed.

Hannibal departed Cartagena, Spain (New Carthage) in late spring of 218 BC. He fought his way through the northern tribes to the foothills of the Pyrenees, subduing the tribes through clever mountain tactics and stubborn fighting. He left a detachment of 20,000 troops to garrison the newly conquered region. At the Pyrenees, he released 11,000 Iberian troops who showed reluctance to leave their homeland. Hannibal reportedly entered Gaul with 40,000 foot soldiers and 12,000 horsemen.

Hannibal recognized that he still needed to cross the Pyrenees, the Alps, and many large rivers. Additionally, he would have to contend with opposition from the Gauls, whose territory he passed through. Starting in the spring of 218 BC, he crossed the Pyrenees and, by conciliating the Gaulish chiefs along his passage before the Romans could take any measures to bar his advance, was able to reach the Rhône by September. Hannibal's army numbered 38,000 infantry, 8,000 cavalry, and 38 elephants, almost none of which would survive the harsh conditions of the Alps.

Hannibal outmanoeuvred the natives who had tried to prevent his crossing, then evaded a Roman force marching from the Mediterranean coast by turning inland up the valley of the Rhône. His exact route over the Alps has been the source of scholarly dispute ever since (Polybius, the surviving ancient account closest in time to Hannibal's campaign, reports that the route was already debated). The most influential modern theories favour either a march up the valley of the Drôme and a crossing of the main range to the south of the modern highway over the Col de Montgenèvre or a march farther north up the valleys of the Isère and Arc crossing the main range near the present Col de Mont Cenis or the Little St Bernard Pass.

Recent numismatic evidence suggests that Hannibal's army passed within sight of the Matterhorn. Stanford geoarchaeologist Patrick Hunt argues that Hannibal took the Col de Clapier mountain pass, claiming the Clapier most accurately met ancient depictions of the route: wide view of Italy, pockets of year-round snow, and a large campground. Other scholars have doubts, proposing that Hannibal took the easier route across Petit Mount Cenis. Hunt responds to this by proposing that Hannibal's Celtic guides purposefully misguided the Carthaginian general.

Most recently, W. C. Mahaney has argued Col de la Traversette closest fits the records of ancient authors. Biostratigraphic archaeological data has reinforced the case for Col de la Traversette; analysis of peat bogs near watercourses on both sides of the pass's summit showed that the ground was heavily disturbed "by thousands, perhaps tens of thousands, of animals and humans" and that the soil bore traces of unique levels of Clostridia bacteria associated with the digestive tract of horses and mules.

Radiocarbon dating secured dates of 2168 BP or c. 218 BC, the year of Hannibal's march. Mahaney et al. have concluded that this and other evidence strongly supports the Col de la Traversette as being the "Hannibalic Route" as had been argued by Gavin de Beer in 1954. De Beer was one of only three interpreters—the others being John Lazenby and Jakob Seibert—to have visited all the Alpine high passes and presented a view on which was most plausible. Both De Beer and Siebert had selected the Col de la Traversette as the one most closely matching the ancient descriptions.

Polybius wrote that Hannibal had crossed the highest of the Alpine passes: Col de la Traversette, between the upper Guil valley and the upper Po river is the highest pass. It is moreover the most southerly, as Varro in his De re rustica relates, agreeing that Hannibal's Pass was the highest in Western Alps and the most southerly. Mahaney et al. argue that factors used by De Beer to support Col de la Traversette including "gauging ancient place names against modern, close scrutiny of times of flood in major rivers and distant viewing of the Po plains" taken together with "massive radiocarbon and microbiological and parasitical evidence" from the alluvial sediments either side of the pass furnish "supporting evidence, proof if you will" that Hannibal's invasion went that way. If Hannibal had ascended the Col de la Traversette, the Po Valley would indeed have been visible from the pass's summit, vindicating Polybius's account.

By Livy's account, the crossing was accomplished in the face of huge difficulties. These Hannibal surmounted with ingenuity, such as when he used vinegar and fire to break through a rockfall. According to Polybius, he arrived in Italy accompanied by 20,000 foot soldiers, 4,000 horsemen, and only a few elephants. The fired rockfall event is mentioned only by Livy; Polybius is mute on the subject and there is no evidence of carbonized rock at the only two-tier rockfall in the Western Alps, located below the Col de la Traversette (Mahaney, 2008). If Polybius is correct in his figure for the number of troops that he commanded after the crossing of the Rhône, this would suggest that he had lost almost half of his force. Historians such as Serge Lancel have questioned the reliability of the figures for the number of troops that he had when he left Hispania. From the start, he seems to have calculated that he would have to operate without aid from Hispania.

Hannibal's vision of military affairs was derived partly from the teaching of his Greek tutors and partly from experience gained alongside his father, and it stretched over most of the Hellenistic World of his time. The breadth of his vision gave rise to his grand strategy of conquering Rome by opening a northern front and subduing allied city-states on the peninsula, rather than by attacking Rome directly. Historical events that led to the defeat of Carthage during the First Punic War when his father commanded the Carthaginian Army also led Hannibal to plan the invasion of Italy by land across the Alps. The task involved the mobilization of between 60,000 and 100,000 troops and the training of a war-elephant corps, all of which had to be provisioned along the way. The alpine invasion of Italy was a military operation that would shake the Mediterranean World of 218 BC with repercussions for more than two decades.