The Helvetii were a tribe that originated from what is modern day Switzerland, who, just prior to the battle with Caesar, had commenced on a mass migration through Roman Gaul towards the Atlantic coast.
At Geneva, the Romans destroyed the wooden bridge across the Rhone and constructed nineteen miles of fortifications. The Helvetii tribe tried to migrate by another route, and were crossing the river Arar (Saône) using rafts and boats. Caesar was informed by his scouts and proceeded to engage the Helvetii. Three parts of the Helvetii forces had crossed the river and Caesar routed the fourth part left on his side of the river, killing a great many and driving the rest into the woods.
Peace negotiations having failed, the Helvetii resumed their migration with the Romans following close behind. After fifteen days of pursuit Caesar, short of supplies, decided to make a diversion to Bibracte. The Helvetii attacked the Romans, but suffered a decisive defeat.
The Helvetii Caesar defeated were part of the pagus (sub-tribe) of the Tigurini, which in 107 BC had slain the Consul Lucius Cassius Longinus, as well as the legate Lucius Calpurnius Piso, the grandfather of the Lucius Calpurnius Piso who was the father-in-law of Caesar.