A Celtic tribe living in modern day Switzerland, the Helvetii, were at the time under pressure themselves from various Germanic tribes in the area. Under their chieftan Orgetorix, they had planned a move from the Alps region to the west of modern France, or Aquitania. In order to make such a move, however, the Helvetians would have to march not only through Roman controlled territory, but that of the Roman allied Aedui tribe as well. Other Gallic Celts and people within the province of Gallia Narbonensis feared that the Helvetii wouldn't just move through as they proposed, but would plunder everything in their path as they went. Without question, Caesar opposed the idea and hastily recruited two more fresh legions in preparation.
Before the Helvetii marched however, Orgetorix died, but the planning for the move continued. Several other local tribes joined the Helvetti in lesser numbers making the entire force among the largest and most powerful in all of Gaul. In total, according to Caesar, nearly 370,000 tribesmen were gathered, of which about 260,000 were women, children and other non-combatants. Before leaving, the Helvetii burned their villages and destroyed what foodstuff and other commodities could not be taken along. The intention was to make certain that they continued to their destination against all odds.
After setting off, and disregarding Caesar's objection, the two forces inevitably met. After several skirmishes, Caesar, occupied the high ground with his six legions, and lured the enemy into a poorly matched battle. Somewhere near the Aedui capital of Bibracte, Caesar crushed the Helvetii, slaughtering the enemy wholesale with little regard for combat status. According to Caesar himself, of the 370,000 enemy present, only 130,000 survived the battle. In the next few days following the battle while chasing down the fleeing enemy, it seems that at least another 20,000 were killed. While the numbers may very well have been exaggerated, there is no doubt of the slaughter. In all, nearly 260,000 people including a great many women and children were reportedly killed. While today this may seem an atrocity, to the Roman people these Helvetii, seemingly mistaken for Germanics, were considered the barbaric enemy deserving of no better fate.
Caesar's great victory left him with other problems however. First he forced the Helvetii back to their home land to prevent more Germanic incursions into what had become open land. Next he allowed the somewhat friendly or at least pacified Boii tribe to settle into a buffer zone with the Aedui and the Helvetii. The weakened state of these southern Gallic tribes, though, thanks to Caesar's conquest, left parts of Gaul open to Germanic incursions. A federation of tribal leaders came to Caesar to ask assistance against their old nemesis Ariovistus and his Suebi tribesmen. Certainly not noting the irony of his actions against the Helvetii, Caesar's own words describe the despair of one particular tribe, the Sequani, who faced the raids and occupation of Ariovistus. Seeing the obvious potential for further glory, under the pretense of responding to calls for help, he then took the opportunity to protect his Gallic 'friends'.