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.
Battle of the Allia
The Gallic sack led to a long-lasting and profound fear of the Gauls in Rome. In 350 and 349 BC, unspecified Gauls attacked Latium. They were probably marauding raids. On the second occasion, Marcus Valerius Corvus was said to have fought a duel with a Gallic champion. Polybius said that Rome made a peace with the Gauls, who did not return for 30 years. Despite Rome defeating the Senones in the Battle of Sentinum (295) during the Third Samnite War (298-290), popular fear of the Gauls persisted. In 228, 216, and 114 fears of Gallic attacks led to the Romans performing human sacrifices by burying alive a pair of Gauls and a pair of Greeks even though human sacrifice was not a Roman custom. Presumably, that was to avert the danger of Gallic disaster.