The Second Punic War pitted the Roman Republic against what was then the other great power in the Mediterranean—Carthage, an ancient city located in what is now Tunisia. Carthage controlled the coastline of north Africa and the Iberian peninsula. Rome hoped to use its naval dominance to invade and force its rival to accept Roman terms for peace.

To thwart that threat, Carthage turned to its greatest general, the legendary Hannibal, who came up with a bold plan: He would strike first by invading Italy and forcing the Romans to defend their own territory. But to get to Italy, Hannibal had to pull off one of the most daring feats ever attempted by a military leader—he would need to lead his forces through the Alps.

That meant navigating rugged terrain, snow and ice and getting through mountain passes controlled by fierce, tough local tribesmen who could attack them from above. And Hannibal’s forces were hardly small and nimble —they numbered 70,000 men, 20,000 horses—and 37 elephants.

It was a move fraught with peril that easily could have become one history’s great military disasters. Yet, not only did Hannibal manage to march his army across the Alps, the Carthaginians made the journey in just 16 days, according to the Roman historian Titus Livius (Livy).

“Hannibal had to cross the Alps because he could not move his forces to Italy by sea, he did not have enough allied ports to support that kind of maritime expedition,” says Eve MacDonald, a senior lecturer in ancient history at Cardiff University, and author of the 2015 biography Hannibal: A Hellenistic Life. “He also needed local support in Italy, so to get to the north of Italy through the Alps meant he would be able to rally support from the population in Northern Italy who were very hostile to the Romans already.”

The precise route that Hannibal took through the Alps has been debated for more than 2,200 years. But research based upon ancient deposits of animal excrement, suggests that the Carthaginians may have gone through the Col de la Traversette, a narrow pass in southeastern France along the border with Italy that’s about 7,700 feet in altitude.

When Hannibal’s army finally reached the Alps in late September or early October 218 B.C., winter was fast approaching. There was little time to lose. His soldiers were dressed for the cold and snow, and may have equipped their sandals with the equivalent of modern crampons to give them more traction.

Two centuries later, Livy, who drew upon an earlier Carthaginian account for his narrative, imagined the scene. “The dreadful vision was now before their eyes,” he wrote. “The towering peaks, the snow clad pinnacles soaring to the sky, the rude huts clinging to the rocks, beasts and cattle shriveled and parched with cold, the people with their wild and ragged hair, all nature, animate and inanimate, stiff with frost: all this, and other sights the horror of which words cannot express, gave a fresh edge to their apprehension.”

As the first Carthaginian column warily advanced, tribesmen appeared all around them, according to Livy’s account. Hannibal quickly ordered his troops to halt, and sent his Gallic guides out on reconnaissance. The following morning, he kept most of his army in their camp, while he assembled a group of his bravest, most determined light infantrymen, who stealthily scaled and secured the highest spots they could find along the route. As Livy writes, when the locals showed up the next day, to their shock, “they saw the Carthaginian assault troops right above their heads.”

Nevertheless, as the tribesmen saw what a difficult time that the long trains of men and pack animals in the main Carthaginian force was having with the terrain, they decided to attack anyway. The Carthaginians suffered heavy losses. “The road leading up to the pass was not only narrow and uneven but flanked with precipices, and so the least movement or disorder in the line caused many of the animals to be forced over the edge with their loads,” the Roman historian Polybius wrote.

Hannibal and his men also managed to inflict heavy casualties upon their attackers, and after driving them off, attacked and captured the enemy’s nearby town, where they recovered some soldiers, mules and horses who had been captured, and seized enough food to last several days, according to Polybius’ account.

Soon afterward, Hannibal made peace with the mountain chieftains. But they turned out to be as treacherous as the terrain, launching another surprise attack just as the Carthaginian army was making its way through a particularly difficult part of the pass. Again, Hannibal and his soldiers took heavy losses, but managed to fight off the attackers. Hannibal’s force of elephants, who may have belonged to an extinct subspecies from the Atlas Mountains in Morocco and Algeria, proved a valuable asset. “The enemy were terrified by their strange appearance, and never dared approach the part of the column in which they were stationed,” Polybius wrote.

In addition to the weather, the terrain and the hostile natives, MacDonald thinks that the psychological impact of being in the Alps added to the Carthaginians’ ordeal.

“I think we forget today just how enormous these mountains would have seemed, full of monsters and myths to his army, and impenetrable.” MacDonald says. “His soldiers must have been terrified, the weather was harsh, freezing and snowy."

In addition to toughness, ingenuity helped the Carthaginians to survive the journey. Hannibal himself is given credit by Livy for coming up with a method for cutting new path through a landslide using spoiled wine and fire, though as MacDonald notes, “this may just be part of the myth.”

By the ninth day, Hannibal’s army had reached the highest point of the route, but the men felt exhausted and hopeless, according to Polybius’ account. Seeing their despair, Hannibal rode on his horse to the front. He halted the army and exhorted them to gaze down into Italy’s Po Valley. “Henceforward all will be easy going—no more hills to climb,” the general told his men.

Unfortunately for the Carthaginians, the descent was even steeper and more difficult than the hike into the mountains. In Polybius’ account, the Carthaginians slipped and slid down the slope. “It was impossible for a man to keep his feet,” he wrote, adding “the least stumble meant a fall, and a fall a slide, so that there was indescribable confusion, men and beasts stumbling and slipping on top of each other.”

“The weather was treacherous on the way down and that was the biggest challenge and likely the biggest loss of animal life,” MacDonald says. “The supply chain had been broken by this point, the soldiers and animals would have been starving, the snow and ice made the descent deadly.”

Eventually, the Carthaginians got to the bottom, where it was warmer and the landscape was green. Hannibal had succeeded, at least in the short term. He spent the next several years fighting the Romans on their own territory, repeatedly defeating them and inflicting heavy losses. At the battle of Cannae in 216 B.C., his forces killed at least 50,000 Roman legionaries. But Hannibal never had the resources to enable him to mount an attack upon Rome, possibly because of his family’s strained relationship with the Carthaginian senate back home.

“The failure to reinforce Hannibal in Italy was significant, and Carthage's lack of dominance on the seas was a huge liability,” MacDonald says. “It is fair to argue that once Carthage could not hold on to and defend Iberia from the Romans it meant almost certain defeat of Hannibal in Italy.”

Hannibal eventually was recalled to defend Carthage from the Romans, who defeated him at the Battle of Zama in 202 B.C.  Defeat in the Second Punic War effectively ended Carthage’s challenge to Rome, and a half century later, the Romans destroyed the city and claimed Carthaginian territory as part of the Roman realm. Still, long after the empire he fought to preserve had been largely forgotten, Hannibal is remembered for the audacity of his march through the Alps.