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.
Battle of Alesia
At one point in the battle the Romans were outnumbered by the Gauls by four to one. The event is described by several contemporary authors, including Caesar himself in his "Commentarii de Bello Gallico". After the Roman victory, Gaul (very roughly modern France) was subdued and became a Roman province. The Roman Senate granted a thanksgiving of 20 days for his victory in the Gallic War.