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.
Gauls
The Gauls emerged around the 5th century BC as the bearers of the La Tène culture north of the Alps (spread across the lands between the Seine, Middle Rhine and upper Elbe). By the 4th century BC, they spread over much of what is now France, Belgium, Spain, Portugal, Switzerland, Southern Germany, Austria, the Czech Republic and Slovakia by virtue of controlling the trade routes along the river systems of the Rhône, Seine, Rhine, and Danube, and they quickly expanded into Northern Italy, the Balkans, Transylvania and Galatia. Gaul was never united under a single ruler or government, but the Gallic tribes were 
capable of uniting their forces in large-scale military operations. They reached the peak of their power in the early 3rd century BC.
The rising Roman Republic after the end of the First Punic War increasingly put pressure on the Gallic sphere of influence; the Battle of Telamon of 225 BC heralded a gradual decline of Gallic power over the 2nd century, until the eventual conquest of Gaul in the Gallic Wars of the 50s BC. 
After this, Gaul became a province of the Roman Empire, and the Gauls were ethnically and culturally largely assimilated into Latin (Roman settlers) majority, losing their tribal identities by the end of the 1st century AD.