The battle of Bibracte (June/July 58 B.C.) was the second and decisive battle in Julius Caesar's first military campaign, and saw him force the Helvetii tribe to abandon their planned migration from Switzerland to the west coast of France. The Helvetii left their homelands in late March 58 B.C. At first they had hoped to cross the Roman Province of Transalpine Gaul peacefully, but after playing for time Caesar refused to allow them across the border. The Helvetii were forced to find an alternative route, which took them through the lands of Rome's allies, the Aedui.
While the Helvetii began their march Caesar returned to Italy to gather three veteran and two new legions, then returned across the Alps and prepared to pursue the Helvetii. In June he caught up with them as they were crossing the Saone River, and defeated the Tigurini canton, the last part of migrating horde left on the east bank of the river (battle of the Arar). Caesar then ordered a bridge to be thrown across the river, and all six of his legions crossed the Saone in a single day.
According to Caesar the Helvetii had taken twenty days to cross the Saone, and the rapid Roman crossing convinced them to ask for a meeting. Neither side was willing to give in to the other's demands, and the migration continued, with the Romans following close behind. As was so often the case, the weakest part of the Roman army was the cavalry. Caesar had to rely on 4,000 cavalry raised from the Province, the Aedui and their allies. This force was commanded by Dumnorix of the Aedui, who was opposed to the Roman involvement in Gaul. On the day after the meeting his cavalry was involved in a clash with the Helvetian rearguard which ended with a minor Roman defeat, and greatly encouraged the Helvetii.
For the next fifteen days the Romans followed closely behind the Helvetii, with a gap of no more than five or six miles between the two armies. Caesar's main problem was a lack of supplies. His own supply train was tied to the rivers, and so he was relying on the Aedui to provide him with corn, but none reached the army. Eventually Caesar confronted the Aedui leaders and Liscus, the Vergobretus (the annually elected chief magistrate of the tribe) admitted that Dumnorix was the leader of a faction that was opposed to the Romans, and had been preventing any supplies from reaching the army. Dumnorix's brother Divitiacus persuaded Caesar not to prosecute him, but Dumnorix was placed under observation.
On the same day Caesar discovered a chance to force a battle on very favourable terms. The Helvetii had camped at the foot of a mountain. Overnight Titus Labienus led two legions onto that mountain and was ready to fall on the Gauls, but on the following morning one of Caesar's scouts misidentified the legions as Gauls, and the attack was called off.
After the failure of this attack Caesar decided to make a diversion to Bibracte, the chief and largest town of the Aedui, where he expected to find some supplies. The Romans were eighteen miles away from the town, while was built on Mount Beuvray, fifteen miles to the west of Autun. On the next morning the Romans began to march towards Bibracte. When the Helvetii discovered that the Romans were heading away from them they decided to turn the tables, follow them and harass the Roman rear.
This played into Caesar's hands. When news reached him of the Helvetii pursuit he sent his cavalry to hold them up while the Roman infantry formed up on the next hill on their line of march. Caesar placed his four veteran legions in the front line with the two new legions and the auxiliaries behind and above them on the hilltop. The Helvetii followed, drove off the Roman cavalry and prepared to attack. They formed up in very close order, in what Caesar described as a phalanx, and advanced towards the Romans on their hill.
As the Helvetii advanced towards the Roman lines Caesar ordered his officers horses to be moved out of sight, to make sure that nobody though about retreating. Although the Gauls were advancing towards the Roman lines, according to Caesar it was the Romans who opened the battle, first launching a volley of javelins and then charging into close combat. The Gallic phalanx was quickly broken up, and the Helvetii began to retreat back towards a mountain about a mile to their rear, followed closely by the Romans.
As the Romans advanced they were attacked in the flank and then the rear by the Boii and the Tulingi, who had been acting as the Helvetii rear guard. This encouraged the rest of the army to stop its retreat and return to the battle, and the Romans found themselves under attack from front and rear. Caesar responded by ordering his third line to turn around to face the Boii and Tulingi while the first two lines held off the main Gallic force.
This phase of the battle lasted for most of the afternoon, but towards evening the Gauls were finally forced to retreat. The main force pulled back onto the mountain they had been retreating towards and the Boii and Tulingi retreating into the camp. The two Gallic forces continued to fight into the night, with the fight around the camp lasting longest, but eventually the Romans broke into the camp, ending the battle.
Overnight the 130,000 surviving Helvetti soldiers began a four day march into the lands of the Lingones. The Romans were unable to follow them for the first three days while they recovered from the fighting, but when they did finally move on the Helvetii surrendered. Caesar ordered the Helvetii, the Tulingi and the Latobrigi to return to their homelands, but the Aedui asked him to allow the Boii to settle on Aedui lands. Caesar agreed to this, and the Boii soon became part of the Aedui.
When they searched the Helvetii camp after the battle the Romans found a document that listed the number of people involved in the migration (368,000 in total) and the number of men capable of bearing arms (92,000). According to Caesar a census of those who returned home produced a total of 110,000. Given that only a few paragraphs earlier Caesar states that 130,000 men of the enemy survived the battle, the 110,000 surely must refer to fighting men and not to the entire migration. According the Caesar the Gauls never turned their back during this battle, and so their overall casualties could have been comparatively light.
The defeat of the Helvetii only marked the end of the first phase of the Gallic Wars. After his victory Caesar was joined by a delegation of leading Gauls, who apparently asked him to intervene against the Roman king Ariovistus, who had crossed the Rhine some years earlier at the request of one of the Gallic tribes. Even the defeat of Ariovistus at Vesontio, later in 58 B.C., didn't end the fighting, for Caesar then turned on the Belgae, and a war that had started just outside the borders of the Roman Province soon spread across all of Gaul.