Hannibal's crossing of the Alps in 218 BC was one of the major events of the Second Punic War and one of the most celebrated achievements of any military force in ancient warfare.
Hannibal led his Carthaginian army over the Alps and into Italy to take the war directly to the Roman Republic, bypassing Roman and allied land garrisons and Roman naval dominance.
The two primary sources for the event are Polybius and Livy, who were born c. 20 years and c. 160 years after the event, respectively. The Alps were not well-documented at the time, and no archaeological evidence is available, so all modern theories depend on interpreting the three place names used by Polybius (Island, Skaras and Allobroges) and Livy's wider range of tribe and place names, and comparing them with modern geographical knowledge.
The 2022 book Hannibal in the Alps by Dutch historian and publicist Jona Lendering concludes that the two primary historical sources provide too little accurate information and too much conflicting information, combined with a lack of historical geographical knowledge and current knowledge of historical armies in order to define the route of Hannibal's army over the Alps. French historians have coined the phrase 'Hannibalism' for trying to answer a question that is intrinsically impossible to answer.
Background
After the final Carthaginian naval defeat at the Aegates Islands, the Carthaginians surrendered in the First Punic War. Hamilcar Barca (Barca meaning lightning), a leading member of the patriotic Barcine party in Carthage and a general in the First Punic War, sought to remedy the losses that Carthage had suffered in Sicily to the Romans. In addition to this, the Carthaginians, and Hamilcar personally, were embittered by the loss of Sardinia.
After the Carthaginians' loss of the war, the Romans imposed terms that were designed to reduce Carthage to a tribute-paying city and simultaneously strip it of its fleet. While the terms of the peace treaty were harsh, the Romans did not strip Carthage of its strength; Carthage was the most prosperous maritime trading port of its day, and the tribute imposed by the Romans was easily paid off on a yearly basis, while Carthage was simultaneously engaged by Carthaginian mercenaries who were in revolt.
The Carthaginian Barcine party was interested in conquering Iberia, a land whose natural resources would provide needed revenue and replace the resources of Sicily that, following the end of the First Punic War, were now flowing to the Romans. It was the ambition of the Barcas, one of the leading noble families of the patriotic party, to establish the Iberian peninsula as a base of operations for waging a war of revenge against the Roman military alliance. Those two factors went together, and in spite of conservative opposition to his expedition, Hamilcar set out in 238 BC to begin his conquest of the Iberian peninsula with these objectives in mind. Marching west from Carthage towards the Pillars of Hercules, where his army crossed the strait and proceeded to subdue the peninsula, in the course of nine years Hamilcar conquered the south-eastern portion of the peninsula.
In 228 BC, Hamilcar was killed, witnessed by Hannibal, during a campaign against the Celtic natives of the peninsula. The commanding naval officer, who was both Hamilcar's son in law and a member of the Patriotic party – Hasdrubal "The Handsome" – was awarded the chief command by the officers of the Carthaginian Iberian army. There were a number of Grecian colonies along the eastern coast of the Iberian peninsula, the most notable being the trade emporium of Saguntum. These colonies expressed concern about the consolidation of Carthaginian power on the peninsula, which Hasdrubal's deft military leadership and diplomatic skill procured. For protection, Saguntum turned to Rome; Rome sent a garrison to the city and a diplomatic mission to Hasdrubal's camp in Cartagena, informing him that the Iberus (modern day Ebro) river must be the limit of the Carthaginian advance in Spain. The conclusion of the treaty and the embassy were sent to Hasdrubal's camp in 226 BC.
In 221 BC, Hasdrubal was killed by an assassin. It was in that year that the officers of the Carthaginian army in Iberia expressed their high opinion of Hamilcar's 26-year-old son, Hannibal, by electing him to the chief command of the army. Having assumed the command (retroactively confirmed by the Carthaginian Senate) of the army that his father had wielded through nine years of hard mountain fighting, Hannibal declared that he was going to finish his father's project of conquering the Iberian peninsula, which had been the first objective in his father's plan to bring a war to Rome in Italy, and defeat it there.
Hannibal spent the first two years of his command seeking to complete his father's ambition while simultaneously putting down several potential revolts that resulted in part from the death of Hasdrubal, which menaced the Carthaginian possessions already conquered thus far. He attacked the tribe known as the Olcades and captured their chief town of Althaea. Neighbouring tribes were surprised at the vigour and rapacity of this attack, as a result of which they submitted to the Carthaginians. He received tribute from all of these recently subjugated tribes, and marched his army back to Cartagena, where he rewarded his troops with gifts and promised more gifts in the future. During the next two years, Hannibal reduced all of Iberia south of the Ebro to subjection, excepting the city of Saguntum, which, under the aegis of Rome, was outside of his immediate plans. Catalonia and Saguntum were now the only areas of the peninsula not in Hannibal's possession.
Roman foreign relations
Hannibal was well-informed on Roman politics, and realized that this was an opportune time to attack. He had Gallic spies in every corner of the Roman Republic, even within the inner circles of the Senate itself. The Romans had spent the years since the end of the First Punic War (264–241 BC) tightening their grip on the Italian peninsula by taking important geographical positions, in addition to extending Rome's grip on Sicily, Corsica and Sardinia.
In addition to this, the Romans had been at war with the Padane Gauls off and on for more than a century. The Boii had waged war upon the Romans in 238 BC, a war that lasted until 236 BC. In 225 BC, the natives of northern Italy, seeing that Rome was again moving aggressively to colonize their territory, progressed to the attack, but were defeated. The Romans were determined to drive their borders right up to the Alps. In 224 BC, the Boii submitted to Roman hegemony, and the next year the Anari also submitted to the Romans.
In 223 BC, the Romans engaged in another battle with the Gauls, this time the Insubres. The Romans at first sustained significant losses against the Insubres while they were attempting to cross a ford near the junction of the Po and the Adda. After encamping in this country for some days without taking any decisive action, the Roman consul on the spot decided to negotiate a settlement with the Insubres. Under the terms of this freshly negotiated truce, the Romans marched out with full honours into the territory of their allies, the Cenomani. However, once they were safe within the territory of the Cenomani, the Romans again marched their army into the territory of the Insubres and were victorious.
In 222 BC, the Celts sent an embassy to the Roman Senate, pleading for peace. Seeing an opportunity for a triumph for themselves, the consuls Marcus Claudius and Gnaeus Cornelius vigorously rejected the embassy, and the Gauls prepared for war with the Romans. They hired 30,000 mercenaries from beyond the Alps and awaited the arrival of the Romans. When the campaigning season began, the consular legions were marched into the Insubres territory again. A vigorous combat took place near Mediolanum, which resulted in the leaders of the Gallic revolt turning themselves over to the Romans. With this victory, the Padane Gauls were unhappily subdued, and ripe for revolt.
Preparations
Hannibal, aware of the situation, sent a number of embassies to the Gallic tribes in the Po Valley. In 220 BC, he had begun to communicate intimately with the Padane Gauls, so called because the Po in this era was called the "Padus" by the Romans, and these embassies brought with them offers of money, food and guides to the Carthaginians.
This mission had the specific aim of establishing a safe place for Hannibal to debouch from the Alps into the Po Valley. Hannibal did not know a great deal about the Alps, but he knew enough to know that it was going to be a difficult march. He had some scouts give him reports concerning this mountain chain, and he received reports of the difficulties to be encountered there from the Gauls themselves. He did not desire to cross this rugged mountain chain and to descend into the Po Valley with exhausted troops only to have to fight a battle.
Hannibal knew enough about the Alps to know in particular that the descent was steeper than the ascent into the Alps. This was one of the reasons he wanted to have allies into whose territory he could march.
The Romans had poorly treated those Gauls whom they had recently conquered, distributing their land to Roman colonists and taking other unscrupulous measures to ensure their own security, against the freshly-conquered tribes. The Insubres, whose tribal territory immediately abutted the Alps, and the Boii, farther down the Po, were particularly pleased with Hannibal's proposed invasion. In addition, much of the Iberian Peninsula was populated by related Gallic tribes, and those same Gauls were serving in Hannibal's army. It would be easy indeed to establish intimate relations with these disaffected tribes, especially once he had debouched from the Alps and was amongst them and the Insubres and Boii and other tribes could see and speak with this army for themselves. Polybius had this to say about Hannibal's plans:
Conducted his enterprise with consummate judgement; for he had accurately ascertained the excellent nature of the country in which he was to arrive, and the hostile disposition of its inhabitants towards the Romans; and he had for guides and conductors through the difficult passes which lay in the way of natives of the country, men who were to partake of the same hopes with himself
Siege of Saguntum
These preparations being completed, Hannibal sought to induce the Saguntines to come to arms with him and thereby declare war on Rome through her proxy. He did not desire to break the peace himself, and resorted to a variety of stratagems in order to induce the Saguntines to attack. However, the Saguntines did nothing except send a diplomatic mission to the Romans to complain about the belligerence of the Carthaginians. The Senate sent a committee to Iberia to attempt to settle the issue diplomatically.
Hannibal openly scorned the Roman offer in the hopes that it would drive the commission to declare war. The commission was not fooled and knew that war was in the air. The commission kept its peace, but brought news to Rome that Hannibal was prepared and was going to strike soon. The Senate took a number of measures in order to free up its hands for the coming conflict with the Carthaginian. An Illyrian revolt was put down with energy, and the Romans sped up the construction of a number of fortresses in Cisalpine Gaul. Demetrius of Pharos had abandoned his previous alliance with Rome and was now attacking Illyrian cities that had been incorporated into the Roman State.
Hannibal could not achieve the ends that he had hoped for, and in the end he sent news to Carthage, where the peace party, his political enemies, were in power, to the effect that the Saguntines were aggressively handling one of their subject tribes, the Torboletes, and encamped in front of Saguntum to besiege it without awaiting any reply from Carthage. Words were exchanged in the Carthaginian Senate to the effect that Hannibal should be handed over to the Romans and his actions disavowed. However, the multitude in Carthage was too much in support of the conflict to order a stop to the war.
The siege took place over the course of eight months, and it is notable that the Romans did not send any aid to the Saguntines in spite of this being a part of the terms of their alliance. The Romans allowed themselves to be tied up in a war against the Illyrians, and did not treat the Carthaginian threat from Iberia with the attention that it deserved.
After the siege, Hannibal sold all the inhabitants as slaves, and distributed the proceeds from those sales to his soldiers. In addition, all the booty from the sacking of the city was taken back to Carthage and distributed to the populace, in order to rally their support to his cause. The rest of the city's treasures were put into his war chest for his planned expedition.