The battle of Octodurus (winter 57/56 B.C.) was a battle in the upper Rhone valley described by Julius Caesar as a Roman victory, but that effectively ended an attempt to open the Great St. Bernard Pass.
Octodurus is now Martigny, which is located to the south-east of Lake Geneva, where the River Dranse flows into the Rhone, at a point where the Rhone turns sharply to the north having flown west through the mountains. The Rhone itself flows along the northern side of a relatively narrow plain (well over a mile wide to the east of the town), while Martigny is located on the opposite side of the valley. When Caesar was writing the town was split in two by the Dranse. It is overlooked to the west and south by steep mountains, with more mountains on the opposite side of the Rhone valley. Most importantly the town is located at the northern end of the Great St. Bernard Pass.
In Caesar's time Octodurus was a village of the Veragri tribe, and was on the borders of the Roman Empire. The Great St. Bernard was used by Roman merchants, but only if they paid a large toll to the local Gauls. Having occupied most of the rest of Gaul Caesar decided to send Servius Galba and the twelfth Legion to open up the pass, while he returned to Italy. After conducting a successful campaign against the Nantuates, Veragri and Seduni tribes, Galba decided to go into winter quarters at Octodurus. He occupied one half of the town with all but two cohorts of his legion, and allowed the Gauls to occupy the other half. Work began on fortifying the Roman half of the town, but the fortifications were not complete when the Gauls decided to attack.
Several days after going into winter quarters the Romans woke up to find the Gallic half of the town deserted and the slopes above the town covered with a large force of Seduni and Veragri. Caesar gives a number of reasons for this attack, amongst them a belief that the Romans wouldn't limit themselves to keeping the pass open, anger that a number of their children had been taken as hostages and a belief that the single under-strength Roman legion was vulnerable to attack.
Faced with this unexpected attack Galba held a council of war. His defences were incomplete and there were anyway not enough supplies in the town to resist a siege. A number of his officers suggested that they should attempt to fight their way back out of the valley, but for the moment Galba decided to make a stand in the partially completed fortifications. According to Caesar's account Galba was facing 30,000 Gauls, so was outnumbered by six-to-one (although this figure is clearly only an estimate – 'which number of the barbarians it appeared certain had come up to the camp').
Soon after the council of war the Gauls launched their attack, charging the camp from the heights. The initial attack failed, but the Gauls kept up the pressure for six hours. Caesar's account of the end of the battle demonstrates the value of the experienced junior officers of the Roman Legion. P. Sextius Baculus, a senior centurian, and C. Volusenus, a tribune of the soldiers, approached Galba and suggested that their only hope was to make a sortie from the camp and attempt to drive off the Gauls
Galba agreed with this idea. At this stage the battle had turned into a duel of missile weapons. Galba ordered his men to stop returning fire, to collect the weapons being thrown at them, and to rest as much as possible. The Romans then attacked from all gates of the camp (normally four), and drove the Gauls off. Caesar states that a third of the 30,000 attackers were killed. If the battle ended with something of a stampede then the Gauls would probably have taken high casualties for this was always the most costly part of an ancient battle.
Although this attack had been driven off Galba didn’t feel strong enough to remain at Octodurus. He was also running short of supplies, and the Alps in winter would have been a bad place to forage for food, especially if the Gauls occupied the mountains. After burning the village Galba marched back into the Roman Province and spent the rest of the winter in the lands of the Allobroges.
Although Galba and his men had escaped from the Gallic trap at Octodurus, the battle did show that Caesar's legions were potentially very vulnerable when they were scattered across the countryside, and the renewal of resistance in the Alps was a sign of trouble to come in the rest of Gaul. Caesar's apparently easy conquest of 58-57 B.C. would take another five years to make safe.