Battle of Alesia
Caesar engaged in the Gallic Wars (58–50 BC), which led to his conquest of Gaul beyond Gallia Narbonensis. When the Helvetii, a federation of tribes from what is now Switzerland, planned a migration to the Atlantic coast through Gaul, Caesar went to Geneva and forbade the Helvetii to move into Gaul. While he went to Gallia Cisalpina to collect three other legions, the Helvetii attacked the territories of the Aedui, Ambarri, and Allobroges, three Gallic tribes, which called for Caesar’s help. Caesar and his Gallic allies defeated the Helvetii. The Gallic tribes then asked for Caesar to intervene against an invasion by the Suebi, a Germanic tribe. Caesar defeated the Suebi. In 57 BC he intervened in intra-Gallic conflicts and marched on the Belgae of northern Gaul. From then on he conquered the Gallic peoples one by one. His successes in Gaul brought Caesar political prestige in Rome and great wealth through the spoils of wars and the sale of war captives as slaves.
Gallic Wars
By 58 BC, the Helvetii were well on their way in the planning and provisioning for a mass migration under the leadership of Orgetorix. Caesar mentions as an additional reason their not being able to in turn raid for plunder themselves due to their location. (De Bello Gallico, I, 2) They planned to travel across Gaul to the west coast, a route that would have taken them through lands of the Aedui, a Roman ally, and the Roman province of Transalpine Gaul.