by later commentators, such as Pomponius Mela, during and after the Roman conquest. Celtic troop types and organization No Celtic group employed a regular military as we would understand it. Organization was according to clan grouping and social class. A war leader's immediate companions were known in Gaulish as *ambaxtoi ("those who accompany") a term which passed into Latin and from which the English ambassador ultimately derives. The earliest Celtic encounter with Romans in 387 BC, was an outright disaster for the Roman Republic. The resulting Battle of the Allia, was a defeat and rout for the Romans, after which all of the city of Rome apart from the Capitoline Hill fell. A confederacy of Gaulish and Celtic tribes led by Brennus of the Senones occupied the city for a time before being driven out. Little or no detail is given of the methods of warfare of these Gauls, except that according to Plutarch some were armed with swords and some were mounted. In 280 BC, a great army of Volcae comprising about 85,000 warriors left Pannonia, split into three divisions, and marched south in a great expedition to Macedon and Central Greece. Under the leadership of Cerethrius, 20,000 men moved against the Thracians and Triballi. Another division, led by another Brennus and Acichorius moved against the Paionians, while the third division, headed by Bolgios, aimed for the Macedonians and Illyrians. Leonnorius split off from the group with a sizable force into Thrace, heading further into Asia Minor where they ravaged the country to the shores of the Hellespont, compelled the city of Byzantium to pay them tribute, and then moved into central Anatolia. Phrygia, the region that these Celtic peoples eventually settled became known as Galatia after them. The Gallic campaign of Brennus, eventually met the fierce resistance of a combined allied Greek Army at the second Battle of Thermopylae. The Celts were able to defeat the Greeks but at great cost. Despite the hollow Celtic victory at Thermopylae, Brennus pushed onwards to attack Delphi. Most scholars deem this Gallic Greek campaign a disaster for the Celts. According to Pausanias this force included large numbers of cavalry, organized in a system called Trimarcisia (from the words *tri- *marko- "three horse") dividing them into teams of three, only two of whom would be mounted at one time. The second Brennus' expedition would have originated in Pannonia and Noricum, the same