The Celtic Invasion of Italy c. 400–170 BCE

At the battle of Allia (390 BCE), the Roman army was destroyed by a confederacy of Celtic tribes, led by the Senones under Brennus. The Celts went on to lay waste to Rome, before being routed by the exiled Roman general Camillus. The germ of future greatness can be seen in their response to the Celtic defeat. Overseen by Camillus, they overhauled their weaponry, military tactics and formations – replacing the rigid phalanx with a more mobile fighting unit, the maniple. In 295 BCE, the Romans decisively defeated an alliance of Senones, Samnites, Etruscans and Umbrians at Sentinum. In 283 BCE, the Boii and Senones, in alliance with the Etruscans, were defeated at Lake Vadimo near Arezzo. Thereafter, the Romans were on the offensive. Piacenza and Cremona were Roman colonies founded to subdue the Insubres. After the Celts supported the trans-Alpine invasion of the Carthaginian general, Hannibal, the reconquest was completed with victory at Como (196 BCE).