Celtic warfare
The "Gallic Wars" were a series of military campaigns waged by the Roman proconsul Julius Caesar against Gallic tribes, lasting from 58 BC to 51 BC. The Romans would also raid Britannia and Germania, but these expeditions never developed into full-scale invasions. The Gallic Wars culminated in the decisive Battle of Alesia in 52 BC, in which a complete Roman victory resulted in the expansion of the Roman Republic over the whole of Gaul. The wars paved the way for Caesar to become the sole ruler of the Roman Republic.
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.