Gauls occupy Rome
In the run to the Gallic occupation of Rome, according to Livius, Camillus was tried by the peoples tribune L.Apuleius, because of the way he had treated the spoils from the conquered city of Veii. More likely: this story is invented, to make him absent when the Roman army is defeated and Rome occupied. Also his glorious return has to be doubted.
Clusium under attack of the Gauls
Sure is a Gallic tribe, the Senons (the French city of Sens claims they came from there), was wandering about in Etruria. They seem to have attacked Clusium (modern Chuisi pr: kjusy). According to Livius Clusium asked Rome for help. Being an Etruscan city it would have been more logical to ask the other Etruscan cities for help; Rome being at a distance of about 140 km (= 4 or 5 days of march. But Rome sent ambassadors to the Gauls anyway.
Museo acheologico Napoli
These were the brothers Fabius Ambustus who would give an example of Roman diplomacy. When the Gauls asked in return for peace, to help them negotiate with Clusium over a piece of land where the Gauls could live, the three Romans joined the forces of Clusium. When Q. Fabius slew a Gallic chieftain he was recognized. This made the Gauls furious and now they turned to Rome and asked the surrender of the three Fabii. Instead of complying, the three were elected consular tribune. Quite an insult. Brennus, the Gallic commander, left Clusium and marched with his army towards Roma. The Roman tribunes organized a levy quite casual and underestimating the dangers. The two armies met near the river Alia.
Now the consular tribunes did wrong everything they could. No fortified camp was built, no entrenchments dug and above all no auspices were held. The Gallic army was in numbers much stronger than the Romans. In order to protect their flanks, the Roman commanders widened their frontline but this meant the phalanx was also thinner. The force of a phalanx is her depth, which keeps the first line of men in place or literally push them forward. Right of this phalanx the place a reserve on a low elevation.
Battle at the Alia
Brennus feared for his left flank and decided to attack the reserve first. This caused the Romans to panic. For making it to Roma they would have to pass the Tiber and the empty city of Veii was nearby. So a large number of fugitives went there. Brennus marched to Rome and found there the gates open. The Romans had decided to retreat onto the Capitol hill and defend themselves there. Many fled to neighbouring cities. The old and weak were left to be murdered.
Woe to the vanquished
The Gauls occupied, looted and burnt the city, but couldn't take the well defendable Capitol. So they decided to besiege the Romans. Once they tried to surprise the Romans by climbing in great silence an unprotected side of the hill. It is said, the geese woke M. Manlius, who woke his comrades and pushed the first Gaul of the hill. Soon others joined him and with rock and stones they made the Gauls run. Yet hunger on both sides made them agree a ransom of 1000 lbs. When the Romans protested the Gauls used unjust weights, Brennus threw his sword onto the scale and exclaimed:"Woe to the vanquished.
Now Camillus returned with the army from Veii just in time to make the Gauls leave without the booty. This is probably an invention to diminish the humiliation.(See Polybius 2.22)