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
Caesar was across the Alps in Italy when he received the news. With only a single legion in Transalpine Gaul, the endangered province, he immediately hurried to Geneva and ordered a levy of several auxiliary units and the destruction of the Rhone bridge. The Helvetii sent an embassy to negotiate a peaceful passage, promising to do no harm. Caesar stalled the negotiations for fifteen days, and used the time to fortify his position with a rampart nineteen miles long and a parallel trench (De Bello Gallico, I, 7 and 8).
When the embassy returned, Caesar refused their request and warned them that any forceful attempt to cross the river would be opposed. Several attempts were quickly beaten off. The Helvetii turned back and entered negotiations with the Sequani, and with Dumnorix of the Aeduans, for an alternate route.(De Bello Gallico, I, 8 and 9).