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.
Gaul
As many as a million people (probably 1 in 5 of the Gauls) died, another million were enslaved, 300 clans were subjugated and 800 cities were destroyed during the Gallic Wars. The entire population of the city of Avaricum (Bourges) (40,000 in all) were slaughtered. Before Julius Caesar's campaign against the Helvetii (present-day Switzerland), the Helvetians had numbered 263,000, but afterwards only 100,000 remained, most of whom Caesar took as slaves.