Brennus (or Brennos) was a chieftain of the Senones, a Gallic tribe originating from the modern areas of France known as Seine-et-Marne, Loiret, and Yonne, but which had expanded to occupy northern Italy.
More important historically was a branch of the above (called Senones, by Polybius), who about 400 B.C. made their way over the Alps and, having driven out the Umbrians, settled on the east coast of Italy from Ariminum to Ancona, in the so-called ager Gallicus, and founded the town of Sena Gallica (Sinigaglia), which became their capital.
In 391 they invaded Etruria and besieged Clusium. 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 (18 July 390) and the capture of Rome. In 387 BC he led an army of Cisalpine Gauls in their attack on Rome. It has been theorized that Brennus is actually a title rather than a name. This is because "Brennus" also appears as the name of a Gallic leader 100 years later. It is also possible that Brennus refers to a god, his name taken by the leader before battle in order to invoke the god's favor and powers.
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. 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 vanquished!"
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. 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. Camillus was hailed by his troops as another Romulus, father of his country 'Pater Patriae' and second founder of Rome.
Some historical accounts say 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. This is plausible as dysentery and other sanitation issues have incapacitated and killed large numbers of combat soldiers up until and including modern times.
It has been theorized that Brennus was working in concert with Dionysius of Syracuse, who sought to control all of Sicily. Rome had strong allegiances with Messana, a small city state in north east Sicily, which Dionysius wanted to control. With Rome's army pinned down by Brennus' efforts Dionysius led a campaign which ultimately failed. Brennus may have been paid twice to sack Rome.
However, the more accepted history (usually citing Livy and Plutarch) finds that Senones marched to Rome to exact retribution for three Roman ambassadors breaking the law of nations (oath of neutrality) in hostilities outside of Clusium. According to this history, the Senones marched to Rome, ignoring the surrounding countryside; once there, they sacked the city for 7 months, and then withdrew. For more information, see the Battle of Allia.
A famous depiction is the academic painting Le Brenn et sa part de butin (1893) by Paul Jamin that shows Brennus viewing his share of spoils (predominantly naked captive women) after the looting of Rome.
In popular culture
Brennus was played by Gordon Mitchell in the 1963 film 'Brennus, Enemy of Rome'.