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.
Military campaigns of Julius Caesar
Plutarch claimed that the army had fought against three million men in the course of the Gallic Wars, of whom 1 million died, and another million were enslaved. 300 tribes were subjugated and 800 cities were destroyed. Almost the entire population of the city of Avaricum (Bourges) (40,000 in all) was slaughtered. Julius Caesar reports that 368,000 of the Helvetii left home, of whom 92,000 could bear arms, and only 110,000 returned after the campaign. However, in view of the difficulty of finding accurate counts in the first place, Caesar's propagandistic purposes, and the common gross exaggeration of numbers in ancient texts, the totals of enemy combatants in particular are likely to be far too high. Furger-Gunti considers an army of more than 60,000 fighting Helvetii extremely unlikely in the view of the tactics described, and assumes the actual numbers to have been around 40,000 warriors out of a total of 160,000 emigrants. Delbrück suggests an even lower number of 100,000 people, out of which only 16,000 were fighters, which would make the Celtic force about half the size of the Roman body of ca. 30,000.. men.