As Rome became master of the Italian peninsula and foreign wars began to become prevalent in Roman expansion strategy, the Senate became aware that these positions abroad could not remain tenable with northern Italy left unsecured. Pushing Rome's northernmost border to the natural boundary formed by the Alps was a practical and largely supported measure.
In 232 the Tribune Gaius Flaminius proposed and passed an agrarian law, ceding land to Legionary veterans and poorer classes of citizens. This land, won from the Gauls at the battle of Lake Vadimanus in 283 BC, represented the territory known as Gallia Cisalpina, or Gaul this side (Roman) of the Alps. Over the next half century Roman colonists poured into previously occupied Celtic territory planting new colonies throughout. The extension of the Flaminian Way angered the resident Gauls as this new northern road opened all of northern Italy to Rome resulting in an even heavier increase in Roman colonization. A majority of local tribes, (mainly the Insubres) as well as the Gaesatae from Gallia Transalpina (Gaul across the Alps), were eventually pushed to the point of open military resistance.
In Rome and throughout Italy, remembering the sacking of Rome in the last major Gallic invasion (390? BC), the people were understandably frightened. Fortunately for them, some local tribes who had maintained better relations with Rome since that event; played the intelligence role and kept the Senate informed of Celtic activity. For nearly 10 years, military action was reserved in the Alps region, on a mostly minor scale. By 225 BC, however, the Gauls had sufficiently mustered enough forced to invade Roman territory and threaten the city of Rome once more. As many as 70,000 tribal warriors pushed through Etruria ravaging the country as it had done a century and a half before. The Romans though, having become hardened from its recent large scale wars with Carthage and Illyria, were ready.
Using superior legionary and tactical strategies the Romans surrounded the Gallic invaders near Telamon in 222 BC. Under the command of Consul Marcus Claudius Marcellus, two Roman armies crushed the invasion and put an end to the threat, for the time being. Plutarch, in his "Lives", tells us that Marcellus himself won the day by becoming the third and final Roman commander in history to slay a foreign king in single combat, Britomartus of the Gaesatae. "The first was Romulus, after having slain Acron, king of the Caeninenses: the second, Cornelius Cossus, who slew Tolumnius the Etruscan: after them Marcellus, having killed Britomartus king of the Gauls; after Marcellus, no man."
After the defeat at Telamon, the Gauls were driven back to the Valley of the Po and to the Alps. Within a short time the war ended with Rome victorious having new lands secured and ready for occupation. Three military colonies were established to hold the Gauls at bay; Placentia and Cremona in Insubrian territory and Mutina in that of the Boii. The Via Flaminia was extended from Spoletium to Ariminum a vital part of Roman expansion. As an added benefit to Roman expansion, the loyalty of Italian farmers who settled these new lands increased immeasurably and Roman authority in Italy was further secured.
Hannibal and the Second Punic War would again place this authority in jeopardy, however. As Hannibal crossed the Alps on his way to Italy, his success stirred the Gauls once more and many joined him against Rome. Cisalpine Gaul wouldn't be fully pacified until Hannibal's defeat; and the rest of Gallia Transalpina until the arrival of Caesar in the mid 1st century BC.