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.
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.