Gauls
In 58 BC Julius Caesar launched the Gallic Wars and had conquered the whole of Gaul by 51 BC. He noted that the Gauls (Celtae) were one of the three primary peoples in the area, along with the Aquitanians and the Belgae. Caesar's motivation for the invasion seems to have been his need for gold to pay off his debts and for a successful military expedition to boost his political career. The people of Gaul could provide him with both. So much gold was looted from Gaul that after the war the price of gold fell by as much as 20%. While they were militarily just as brave as the Romans, the internal division between the Gallic tribes guaranteed an easy victory for Caesar, and Vercingetorix's attempt to unite the Gauls against Roman invasion came too late. After the annexation of Gaul a mixed Gallo-Roman culture began to emerge.
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.