The Gauls
Clusium was reached by the Gauls, who had invaded most of Etruria already, and its people turned to Rome for help. However, the Roman embassy provoked a skirmish and, then, the Gauls marched straight for Rome (July, 387 BC). After the entire Roman army was defeated at the Allia brook (Battle of the Allia), the defenseless Rome was seized by the invaders. The entire Roman army retreated into the deserted Veii whereas most civilians ended at the Etruscan Caere. Nonetheless, a surrounded Roman garrison continued to resist on the Capitoline Hill. The Gauls dwelt within the city, getting their supplies by destroying all nearby towns for plunder.
When the Gauls went for Ardea, the exiled Camillus, who was now a private man, organized the local forces for a defense. Particularly, he harangued that, always, the Gauls exterminated their defeated enemies. Camillus found that the Gauls were too distracted, celebrating their latest spoils with much crapulence at their camp. Then, he attacked during a night, defeating the enemy easily with great bloodshed.
Read more about this topic: Marcus Furius Camillus