The Galatians were a confederacy consisting of three Celtic tribes, identified as the Tectosages, the Trocmi and the Tolistobogii, who, in 278 BCE, were transported across the Hellespont from Thrace to Anatolia by King Nicomedes I of Bithynia, who required their assistance in a civil war against his brother, Zipoetes. After helping the Bithynian ruler to vanquish his rival claimant, the Galatians, under the leadership of the joint warlords, Lutarios and Leonnorios the force of some 20,000 Celts, roughly half of which were actual fighting men, the other being their women and children, would travel through the lands of Anatolia and began demanding tribute from Greek city-states in the western Anatolian lands of Lycia, Mysia and Lydia. They sacked the cities who refused their demands.
Shortly after their arrival in Anatolia, the Galatian tribes would settled in the highlands between Phrygia, Lycaonia and Cappadocia, in what would later be known as Galatia. The Celtic newcomers would live alongside the rural Phrygian population and receive tribute from them. The Galatians would generally live apart in separate hill-forts or Duns outside the main cities in the area. The Tectosages would establish their tribal centre on the site of Ancyra (modern Turkish capital of Ankara). The Tolistobogii would maintain their rule around Pessinus, the centre of the cult of Cybele. And the Trocmi would dominate the country around the city of Tavium.
The Galatians tribes, which were each divided into about twelve smaller septs, were each ruled by a chieftain, who was described in Greek sources as 'Tetrarchs'. The tetrarchs, who were assisted by priestly magistrates, who may have been druids, would meet at the Drunemeton, which was held at an undisclosed location. This 'Koinon Galaton', as described by Greek writers such as Strabo, was a multi-tribal commonwealth which remained unified in the face of the surrounding Hellenistic kingdoms, such as Pontus, Bithynia, Pergamum, Cappadocia and the Seleucid Empire, and conduct raids into their lands. As well as occasionally send out smaller warbands to serve as foreign mercenaries as far away as Ptolemaic Egypt. Galatika or Gallic tribute, would enter the Greek language, as the Galatians preyed on smaller Greek and Asian communities.
The Galatians would be defeated in war by by the Seleucid Emperor Antiochus I in 275 BCE, who used Indian elephants, which they had never before seen. However, their settlement in Anatolia continued to be tolerated by the Hellenistic states, who employed them as mercenaries in thier frequent conflicts. The Galatian presence in the highlands of eastern Phrygia and Cappadocia brought in further colonists from the Volcae and Boii tribes in central and western Europe. The most frequent rivals of the Galatians was the Greek kingdom of Pergamon under the Attalid dynasty, who dominated the Greek-speaking lands of Mysia, Lydia and Ionia in western Anatolia, keeping the potential growth of the Galatian confederation in check.
After the defeat of the Seleucid forces of Antiochus III Megas at Magnesia by the allied Romans and Pergamenes in 190 BCE, the newly-elected Roman Consul Gaius Manilius Vulso, organized a new military campaign against the Galatians, who had acted as the allies of the Seleucids. Galatians, since their arrival in Anatolia less than a century earlier, had grown wealthy from the plunder or forced tribute of their neighbours, not to mention hiring out their services to various kingdoms in Asia and the eastern Mediterranean as soldiers of fortune. Vulso is believed to have orchestrated this conflict to enhance his own wealth and reputation in the Republic of Rome. In fact, Vulso had declared war against the Galatians without first consulting the Senate in Rome, setting a dangerous precedent for future Roman generals. Attalus, the regent of Pergamon on behalf of his brother, King Eumenes II (who was visiting Rome at the time), supplied Vulso's campaign with some fifteen hundred troops.