Two Roman armies had been destroyed near the Lake of Geneva, and it seemed as if a repetition of the disaster of the Allia and the capture of Rome itself might not be impossible.
(1) The first Brennus crossed the Apennines in 391 B.C., ravaged Etruria, and annihilated a Roman army of about 40,000 men on the Allia some 12 m.
Veii is mentioned in connexion with the defeat of the Romans at the Allia in 390 B.C., after which many Roman soldiers fled there, while a project was actually broached for abandoning Rome for Veii, which was successfully opposed by Camillus.
The next great wave of Celts recorded was that which swept down on north Italy shortly before 400 B.C. These invaders broke up in a few years the Etruscan power, and even occupied Rome herself after the disaster on the Allia (390 B.C.).
The Clusines appealed to Rome, whose intervention, accompanied by a violation of the law of nations, led to war, the defeat of the Romans at the Allia (18th of July 390) and the capture of Rome.