Brennus, Celtiberia Chief
Most of Western Europe from the 5th Century BC to the 1st Century was a pastiche of fiefdoms and tribal bands and clans and allegiances. The Gauls or the Celtic people, as the Romans referred to them were a widely diverse group that for some reason, began raiding Roman Imperial borders. We know some Barbarians from northern Europe fled into Transalpine Gaul chased by Huns or Tartars or Parthians. It could have just been the desire for self rule or the tempting treasures that a marauding army could purloin at the business end of a sword. Whatever the cause, starting in the 5th century, Celtic tribes began populating the Po River Valley all the way to the Adriatic Coast and Ancona. These were frightening times for Rome, and formative time, that forced the Romans to create a solid expeditionary army that could resist the incursions of Celts along the Iberian Peninsula and into Italy itself. In 390 BC Brennus led Celts to sack the nascent Rome city-state. Rome counter attacked and emerged triumphant, but the tides of war with the Celts shifted back and forth until the Roman Republic began to pay them as mercenaries to be used against other invading people.
In the third century BC Rome had extended warfare with the Samnites, the Umbrians and the Etruscans, but emerged still stronger.
The First Punic War from 264 BC to 241 BC saw the Northern African Empire of Carthage, under the command of Hamilcar Barca raid and invest forces in Spain. Famously using theior own mercenaries, the Barcid Army lost to Rome, and immediately faced a revolt of Gallic mercenaries who fought along side of them. Hamilcar crushed the revolt but it was his son Hannibal that sought revenge on Rome in the Second Punic War.
When Hannibal attacked Seguntum, both sides were using Gallic tribesmen who were fighting for pay many times against each other. In fact, the end of the Second Punic War saw Hannibal defeated at Zama in North Africa when 20,000 Celtic Warriors abandoned the battlefield and paved the way for a win for Scipio and Publius. Then again, there never were enough Carthaginians to defeat Rome. The Barcid family had to employ mercenaries and they financed them by exploiting the Spanish silver mines.
While the Third Punic War saw the Romans erase Carthage from history, the patient Roman Legions took their time and turned their attention to smashing an insurgency by the Boii, Insubres and the Cenomanies. Allegiances changed on the Roman frontier constantly. Once fighting alongside Rome, a tribe would turn tail and join the opposition for money. Viriathus, a Celtiberian leader was able to bring many Celts under one standard and resist Roman expansion. The problem was that Rome was also under the Roman Standards and so after a single defeat the Celtic mercenary tribes had little fidelity beyond their own bands and generally disintegrated. So organization wins out.