The Roman conquest of the Celts began with the breakdown of the peace agreed with the Cisalpine Gauls in 334 bc. Given the nature of Celtic society and the needs of its warrior elite, this was almost inevitable, but there was also a new factor involved. By the end of the fourth century it was obvious to all that Rome was becoming a power to reckon with, and this began to make friends of old enemies among the other peoples of peninsular Italy. In 299 Etruscans joined the Gauls in raiding Roman territory and two years later the Samnites Joined in with them too. In 295 the Romans faced, and defeated, an even greater alliance of the Senones, Etruscans, Umbrians and Samnites in a hard-fought battle at Sentinum in Umbria, but not before the Senones had wiped out a Roman legion at Clusium (Chiusi). Livy describes the triumphant Celts carrying severed heads hanging around their horses’ necks or on the points of spears and singing victory songs. Another victory over a Roman army at Arettium (Arezzo) in 284 made the Senones overconfident and they foolishly and provocatively murdered ambassadors sent from Rome to negotiate terms for the release of prisoners. This made a Roman counterattack inevitable, and this time it was the Senones who were defeated and conquered. A Roman military colony was founded at Sena Gallica (Senigallia) in 283 to prevent any rebellions. The neighbouring Boii tried to liberate the Senones but they too were soundly defeated and negotiations were followed by a long peace.
The key to Roman success against the Gauls was a change in their military organisation and battlefield tactics. At the time of the sack of Rome, the Romans fought in the Greek style, as a phalanx of heavily armoured spearmen. Fighting in a phalanx was the antithesis of the Celtic way of war, as it required strong discipline and teamwork rather than individual heroics. But the phalanx proved to be a dangerously inflexible formation when faced with the more irregular tactics employed by the Celts and it was especially vulnerable to outflanking. The Romans, therefore, abandoned the phalanx and began to develop more flexible tactics based on smaller units of 60 heavily armed infantrymen called maniples (literally ‘handfuls’) supported by light infantry and cavalry. The emphasis on teamwork and discipline remained. The army was organised into legions of3,000-4,000 men (originally legion simply meant ‘levy’ and applied to the whole Roman army). Of great significance, too, was the replacement of the spear as the main weapon by the Javelin. Unlike the Romans, whose heavy infantry were well armoured, most Celts fought with only a shield for protection. This made them very vulnerable to attack with Javelins. A Celtic warrior might successfully stop ajavehn with his shield, but if the Javelin stuck in the shield it became heavy and unwieldy and had to be thrown away, leaving him even more vulnerable.
Conflict between the Gauls and Romans again broke out in 225. A decision by the Romans in 232 to seize the land of the conquered Senones and divide it between the Roman poor alarmed the Boii. In alliance with the Insubres, Taurisci and the Gaesati (‘spear-men’: Celtic mercenaries
From the Alpine regions), the Boii invaded Roman territory with a huge army, claimed by later Roman writers to have included 50,000 infantry and 20,000 cavalry and charioteers. Over 50 years had passed since the last major conflict between the Cisalpine Gauls and the Romans. In that time Rome had completed the conquest of peninsular Italy and captured Sicily, Sardinia and Corsica from Carthage in the First Punic War, making it the major power of the western Mediterranean. Even without its Italian dependencies, Rome could now raise nearly four times as many troops as the Gauls (in 225 Rome had 250,000 citizens qualified to serve as infantry and another 23,000 who could serve as cavalry), and, because the state now usually provided arms and armour, they were equipped to a much higher standard than the average Celtic warrior. The invasion initially went well for the Gauls: they defeated one Roman army at Fiesole, near Florence, and captured an enormous quantity of plunder, prisoners and cattle. When only three days’ march from Rome, the Gauls learned that a second Roman army was approaching from the south, so they began an orderly withdrawal north along the Mediterranean coast. Unknown to the Gauls (and, at first, to the pursuing Roman army too), another Roman army, which had been hurriedly withdrawn from Sardinia, had landed at Pisa to their north and cut off their line of retreat. At Telamon, near the coast of Etruria 100 miles (160 kilometres) north of Rome, the Celtic army was trapped between these two Roman armies and annihilated after heavy fighting. Roman javelins caused great execution among the poorly armoured Gauls, quickly breaking up their charges. The shorter Roman thrusting swords also proved more deadly in close combat than the long Celtic slashing swords. The Gaesati fought completely naked, hoping to intimidate the Romans with their fine physiques, and they suffered dreadful casualties as a consequence. The Gaulish chariots played little part in the battle and there is no further record of their use on the continent after Telamon. The dead included a Celtic king and a Roman consul, whose head was taken as a trophy by the surviving Gauls. Another king, Aneroestes, took his own life. The Romans immediately seized the initiative and began the complete subjection and annexation of Cisalpine Gaul. The Gauls were vulnerable to Roman attack, as many of them had settled in the former Etruscan cities. The conquest took only three years and was completed with the capture of Mediolanum (Milan), the main settlement of the Insubres, in 222. Even the Cenomani, who had taken no part in the battle of Telamon, were conquered. The Romans began to consolidate their conquest by founding military colonies at Piacenza and Cremona, but these had hardly been established when, in 218, the Second Punic War broke out with Carthage.
Recognising that Carthage could not win a long war of attrition, its leading general Hannibal decided on an invasion of Italy in the hope of persuading the Italian peoples to rebel against Rome, so depriving it of most of its manpower. Hannibal took the Romans by surprise by marching his army overland from Spain and crossing the Alps to descend on the valley of the river Po. Hannibal’s army already included a large contingent of Celtiberians and he was immediately greeted as a liberator by the Boii
And the Insubres, who joined his army in their thousands. Hannibal was one of history’s great battlefield commanders, yet, despite a string of spectacular victories, culminating in the battle of Cannae in 216 in which over 30,000 Roman soldiers died, the mass defections he had hoped for never materialised. Most of Italy, including many of the Gauls even, remained loyal to Rome. After Cannae, the Romans wisely gave up trying to fight Hannibal on his own terms and avoided set-piece battles. Using irregular harassing tactics, the Romans gradually pinned Hannibal down in a strategic cul-de-sac in southern Italy while they concentrated their forces on destroying Carthage’s empire in Spain, whose defence was in the hands of Hannibal’s less able brother Hasdrubal. As the war dragged on with no prospect of victory in sight, the Gauls began to desert Hannibal: he had, in any case, always regarded them as disposable ‘cannon fodder’. Carthage finally surrendered to Rome in 201 and the Romans returned to the business of subduing Cisalpine Gaul. Serious resistance to Roman rule ended with the surrender of the Boii in 191, though there was one last invasion from over the Alps in 186, sent back whence it came three years later. Within a few decades most of Cisalpine Gaul south of the Po had been divided up and given to settlers from peninsular Italy, but the land north of the river was left in Gaulish hands. The Alpine tribes still gave occasional trouble into the reign of the emperor Augustus (27 bc-ad 14) and many Gauls took part in the Italian slave rebellion led by the Thracian gladiator Spartacus in 73 bc. The Celtic warrior tradition of single combat made captured Gauls ideal candidates for the gladiatorial schools, and Spartacus’ two most trusted lieutenants, Crixus and Oenomaus, were both Gauls. The rebellion shook Roman Italy to its core, but too many of the participants had no goals beyond plunder and vengeance: despite Spartacus’ fine generalship, the revolt was crushed in 71 bc.