Brennus (or Brennos) was a chieftain of the Senones.
He defeated the Romans at the Battle of the Allia (ca 390 BC).
Later that year, he led an army of Cisalpine Gauls in their attack on Rome and captured most of the city, holding it for several months.
Brennus' sack of Rome was the only time in 800 years the city was occupied by a non-Roman army before the fall of the city to the Visigoths in 410 AD.
In the Battle of the Allia, Brennus defeated the Romans and entered the city itself.
The Senones captured the entire city of Rome except for the Capitoline Hill, which was successfully held against them.
According to legend Marcus Manlius Capitolinus was alerted to the Gallic attack by the sacred geese of Juno.
However, seeing their city devastated, the Romans attempted to buy their salvation from Brennus.
The Romans agreed to pay one thousand pounds weight of gold.
According to Livy, during a dispute over the weights used to measure the gold (the Gauls had brought their own, heavier-than-standard), Brennus threw his sword onto the scales and uttered the famous words "Vae victis!", which translates to "woe to the conquered!".
One version of the story states that the argument about the weights had so delayed matters that the exiled dictator Marcus Furius Camillus had extra time to muster an army, return to Rome and expel the Gauls, saving both the city and the treasury, and telling Brennus, "Non auro, sed ferro, recuperanda est patria", which translates to "not by gold, but by iron, is the nation to be recovered".
According to Plutarch, following initial combat through Rome's streets, the Gauls were first ejected from the city, then utterly annihilated in a regular engagement eight miles outside of town on the road to Gabbi.
Livy says that the Senones besieging the Capitoline Hill were afflicted with an illness and thus were in a weakened state when they took the ransom for Rome.
Silius Italicus claims that Hannibal's Boii cavalrymen were led by a descendant of Brennus named Crixus, who fell in the Battle of Ticinus.