Brennus was a chieftain of the Senones. The Senones were a Celtic tribe originating from the modern areas of France known as Seine-et-Marne, Loiret, and Yonne, but which had expanded to occupy northern Italy. About 400 B.C. a branch of the Senones 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.

Brennus defeated the Romans at the Battle of the Allia on 18 July 390 BC. In 387 BC he led an army of Senones in their attack on Rome and captured most of the city, holding it for several months. They captured the entire city except for the Capitoline Hill, which was successfully held against them. According to legend Marcus Manlius Capitolinus was alerted to the Celtic attack by the sacred geese of Juno. However, seeing their city devastated, the Romans attempted to buy their salvation from Brennus. They agreed to pay one thousand pounds weight of gold. During a dispute over the weights used to measure the gold (the Celts had brought their own, heavier-than-standard scale), Brennus threw his sword onto the scales and uttered the famous words "Vae victis!", translated it means "Woe to the conquered!".

Brennus's sack of Rome was the only time the city was occupied by a non-Roman army before the fall of the city to the Goths in 410 AD.
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