Which pass did Hannibal take?

Carthage, on the coast of what is now Tunisia, fought Rome three times for control of the western Mediterranean. In the second of those wars Hannibal marched to Italy overland. What is known of the march comes from two ancient historians. Polybius was a Greek who wrote within about seventy years of it, questioned men who had been on it, and went over the Alps himself to see the ground. Livy was a Roman who wrote two centuries later and drew on the same Greek tradition. Between them they tell the following story.

Hannibal left New Carthage in Spain with an army that Polybius, from an inscription Hannibal himself set up, puts at forty-six thousand men, with thirty-seven elephants. From the crossing of the Rhône he marched four days upstream to a district the Greeks called the Island, a fertile triangle of land between the Rhône and a second river. Two brothers were quarrelling over its throne. Hannibal backed the elder, who repaid him with weapons, clothing and boots for the mountains and escorted the army with his own troops as far as the foot of the Alps, because the Carthaginians were uneasy about the next tribe on the road, the Allobroges. Ten days along the river brought the army to the first climb. The Allobroges held the heights above the defile by day and went home to their town at night; Hannibal occupied the heights after dark, the column was mauled in the defile next day, and he took the town, whose stores fed the army for two or three days. Four days on, tribesmen came out with branches and wreaths and offered to guide him. They led the army for two days, then attacked it in a steep gorge, rolling rocks down from above, and Hannibal spent a night with half his force on a bare white rock. On the ninth day of the climb he reached the summit and camped there for two days. The first snow was falling, and the Pleiades, a star cluster whose autumn setting was a calendar date for ancient farmers, were going down at dawn, which puts the halt in the last days of October or the first days of November. From the top Hannibal pointed his men to the plains of the Po, the flat land of northern Italy, and to the direction of Rome beyond.

The descent was worse than the climb. The path ran narrow and steep along a cliff, and a stretch of about 270 m had fallen away, an old break freshly enlarged. The previous winter's snow still lay there under the new fall. Hannibal camped on the ridge and set the men to building the path up. One day's work got the horses and mules through to pasture; three more days, with the Numidian troops working in relays, got the elephants over, "in a wretched state from hunger". Three days after that the army was on flat ground among the Taurini, the people around modern Turin, whose chief town it took after a three-day siege. The crossing had taken fifteen days and the march from Spain five months. Livy adds one story Polybius does not have: that the rock on the descent was split by fire and vinegar.

Neither historian names the pass. The route has to be worked out from the days, the distances and the landmarks, and the two place-names that would fix it are damaged in the manuscripts. Every modern translation calls the second river of the Island the Isère, which joins the Rhône at Valence; that is a conjecture made by the geographer Cluverius in 1624, and the manuscripts of both authors read something else. The height of the broken ledge, Livy's "1,000 feet", is a Renaissance restoration of a corrupt line. The argument has gone on since Josias Simler raised it in 1574. A count made in 1835 found ninety published opinions spread over five passes; Colonel Perrin added the Col du Clapier in 1883 by publishing the view from it; Peddie counted twenty-four modern writers in 1997 and found six for the Mont-Cenis, six for the Montgenèvre, five for the Clapier, four for the Petit St Bernard and three for the Col de la Traversette.

A pass, or col, is a gap in the chain low enough for a track to cross, and the candidates lie along the whole frontier between France and Italy. The sides divide first on the river. If the Island's river is the Isère, the army marched north-east up the Isère valley and crossed into the Susa valley west of Turin, by the Mont-Cenis or by the Col du Clapier beside it. The Clapier has been the usual answer, with a hedge, of ancient historians writing in English since Perrin; the Mont-Cenis was Napoleon's choice. If the Island lay further down the Rhône at Orange, as Livy's list of tribes implies, the army marched east up the Durance and crossed by the Montgenèvre, the easiest road into the Susa valley, by the Col de Larche further south, or by the Col de la Traversette, a high notch under Monte Viso at the head of the Po. The Traversette is the highest and hardest of the candidates and has had the most recent support: Gavin de Beer's books, a decade of earth-science papers led by William Mahaney, whose team cored a boggy meadow below the col and reported a layer churned by animals which they dated to 218 BC and identified as the passage of Hannibal's army, and a study of the energy cost of each route published in 2026. The two St Bernard passes to the north, which come down into the Aosta valley, carry the old Roman tradition that Livy himself rejected. No candidate has a majority, and the two most recent peer-reviewed contributions, one for the Clapier and one for the Traversette, point in opposite directions.

I read Polybius (book 3, chapters 39 to 60, and 34.10) and Livy (book 21, chapters 22 to 39) in Greek and Latin, in the standard Teubner and Oxford editions, which record what the manuscripts say, tabulated every number and place in one against the other, and traced the disputed readings to the editors who made them. I drew twelve routes over eight cols on a 90 m terrain model, measured them in stades (the Greek unit, about 180 m) and tested each against Polybius's day-counts; profiled each descent for ground steep and broken enough to match his cliff path; computed by sight-lines what each col can see of the Po plain; compared Polybius's old snow with the modern snowline and the glacier record; and computed the Pleiades' setting for 218 BC. I recalibrated every published radiocarbon date from the Traversette meadow and checked the rest of that team's claims against what their papers report. Then I gave each of fourteen lines of evidence a likelihood range for each of eight passes, argued item by item, and combined them. The nine items that come from Polybius count at half strength in the headline run because they come from one author, and the model was also run at full strength, with each source collapsed to one item, with each item dropped in turn, under seven sceptical scenarios, and by sampling inside every range.

The result ranks the candidates but does not identify the pass. The Col du Clapier comes first at about 40 per cent, the Col de la Traversette second at about 30 and the Mont-Cenis third at about 15. The ranking depends on the Isère reading of the Island: with the Island at Orange the Traversette leads at about 54 per cent. The physical evidence claimed for the Traversette is consistent with any large body of animals at the meadow between about 380 and 150 BC and does not test the crossing. Each percentage rests on the likelihood ranges I set, and the panel below shows how far it moves when a range is changed.

The page begins with the two narratives, Polybius's and Livy's, read in the original with their manuscript variants, from the march to the foot of the Alps to the descent into the land of the Taurini. It then takes up the Island and the approach; the days and stades of the ascent against real terrain; what each candidate col can see of the Po plain; and the snow and the setting of the Pleiades. A short section collects what the other ancient writers say. The Traversette meadow and its radiocarbon dates have a section of their own. Then come the eight candidates with the people who have argued for each, the weighing of the evidence and how its numbers move, what would overturn the result, what is new here and what is not, where the page falls short, and the sources, editions and code.