Helvetii CampaignSaône, France
The Helvetii were a confederation of about five related Gallic tribes that lived on the Swiss plateau, hemmed in by the mountains and the rivers Rhine and Rhône. They had come under increased pressure from Germanic tribes to the north and the east and began planning for a migration around 61 BC. They intended to travel across Gaul to the west coast, a route that would have taken them around the Alps and through lands of the Aedui (a Roman ally) into the Roman province of Transalpine Gaul. As word of the migration spread, neighboring tribes grew concerned, and Rome sent ambassadors to several tribes to convince them not to join the Helvetii. Concern grew in Rome that the Germanic tribes would fill in the lands vacated by the Helvetii. The Romans much preferred the Gauls to the Germanic tribes as neighbors. The consuls of 60 (Metellus) and 59 BC (Caesar) both wanted to lead a campaign against the Gauls, though neither had a casus belli at the time.
On the 28th of March in 58 BC, the Helvetii began their migration, bringing along all their peoples and livestock. They burned their villages and stores to ensure the migration could not be reversed. Upon reaching Transalpine Gaul, where Caesar was governor, they asked permission to cross the Roman lands. Caesar entertained the request but ultimately denied it. The Gauls turned north instead, entirely avoiding Roman lands. The threat to Rome was seemingly over, but Caesar led his army over the border and attacked the Helvetii unprovoked. So began what historian Kate Gilliver describes as "an aggressive war of expansion led by a general who was seeking to advance his career".
Caesar's consideration of the Gallic request to enter Rome was not indecision, but a play for time. He was in Rome when news of the migration arrived, and he rushed to Transalpine Gaul, raising two legions and some auxiliaries along the way. He delivered his refusal to the Gauls, and then promptly returned to Italy to gather the legions he had raised on his previous trip and three veteran legions. Caesar now had between 24,000 and 30,000 legionary troops, and some quantity of auxiliaries, many of whom were themselves Gauls. He marched north to the river Saône, where he caught the Helvetii in the middle of crossing. Some three-quarters had crossed; he slaughtered those who had not. Caesar then crossed the river in one day using a pontoon bridge. He followed the Helvetii, but chose not to engage in combat, waiting for ideal conditions. The Gauls attempted to negotiate, but Caesar's terms were draconian (likely on purpose, as he may have used it as another delaying tactic). Caesar's supplies ran thin on 20 June, forcing him to travel towards allied territory in Bibracte. While his army had easily crossed the Saône, his supply train had still not. The Helvetii could now outmaneuver the Romans and had time to pick up Boii and Tulingi allies. They used this moment to attack Caesar's rearguard.