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 history of France
Around 390 BC, the Gallic chieftain Brennus made his own way through the Alps, defeated the Romans in the Battle of the Allia and sacked Rome for several months. The Gallic invasion left Rome weakened and encouraged several subdued Italian tribes to rebel. One by one, over the course of the next 50 years, these tribes were defeated and brought back under Roman dominion. Meanwhile, the Gauls would continue to harass the region until 345 BC, when they entered into a formal treaty with Rome. But Romans and Gauls would maintain an adversarial relationship for the next several centuries and the Gauls would remain a threat in Italia.