Galatia was named for the Gauls who became its ruling caste in the 3rd century BCE. It has been called the "Gallia" of the East, Roman writers calling its inhabitants Galli. They were an intermixture of Gauls and Greeks, and hence were called Gallo-Graeci, and the country Gallo-Graecia.
The Galatians were in their origin a part of that great Celtic migration which invaded Macedonia, led by the 'second' Brennus, a Gaulish chief. He invaded Greece in 281 BCE with a huge warband and was turned back in the nick of time from plundering the temple of Apollo at Delphi. At the same time, another Gaulish group were migrating with their women and children through Thrace. They had split off from Brennus' Gauls in 279 BCE, and had migrated into Thrace under their leaders Leonnorius and Lutarius. These Gaulish invaders appeared in Asia Minor in 278—277 BCE. As so often happens in cases of invasion, three tribes of Gauls crossed over from Thrace to Asia Minor at the express invitation of Nicomedes I of Bithynia, who required help in a dynastic struggle against his brother. They numbered about 10,000 fighting men and about the same number of women and children, divided into three tribes, Trocmi, Tolistobogii and Tectosages.
The migration led to the establishment of a long-lived Gaulish territory in central Anatolia, which included the eastern part of ancient Phrygia, a territory that became known as Galatia. There they ultimately settled, and being strengthened by fresh accessions of the same clan from Europe, they overran Bithynia, and supported themselves by plundering neighbouring countries.
The Gauls were great warriors, respected by Greeks and Romans. They hired themselves out as mercenary soldiers, sometimes fighting on both sides in the great battles of the times.For years the Gaulish chieftains and their warbands ravaged the western half of Asia Minor, as allies of one or other of the warring princes, without any serious check, until Attalus I of Pergamum (241—197 BCE), inflicted several severe defeats upon them, and about 232 forced them to settle permanently and to confine themselves to the region to which they had already given their name. Their right to the district was formally recognized. The three Gaulish tribes were settled where they afterwards remained, the Tectosages round Ancyra, the Tolistobogii round Pessinus, sacred to Cybele, and the Trocmi round Tavium.
The constitution of the Galatian state is described by Strabo: conformably to Gaulish custom, each tribe was divided into cantons, each governed by a chief ('tetrarch') of its own with a judge under him, whose powers were unlimited except in cases of murder, which were tried before a council of 300 drawn from the twelve cantons and meeting at a holy place, twenty miles southwest of Ancyra, which was likely to have been a sacred oak grove, for it was called 'Drynemeton' the "temple of the oaks" drys + nemed "temple". The local population of Cappadocians were left in control of the towns and most of the land, paying tithes to their new overlords, who formed a military aristocracy and kept aloof in fortified farmsteads, surrounded by their bands.
But the power of the Gauls was not yet broken. They proved a formidable allies of Antiochus, and after Attalus’ death their raids into W. Asia Minor forced Rome in 189 B.C. to send an expedition against them under Ca. Manlius Vulso, who defeated them. The theme of the Dying Gaul remained a favorite in Hellenistic art for a generation. Henceforward their military power declined and they fell at times under Pontic ascendancy, from which they were finally freed by the Mithradatic wars, in which they heartily supported Rome.
In the settlement of 64 B.C. Galatia became a client-state of the empire, the old constitution disappeared, and three chiefs (wrongly styled “tetrarchs “) were appointed, one for each tribe. But this arrangement soon gave way before the ambition of one of these tetrarchs, Deiotarus, the contemporary of Cicero and Caesar, who made himself master of the other two tetrarchies and was finaily recognized by the Romans as 'king' of Galatia. On the death of the third king Amyntas in 25 BCE, however, Galatia was incorporated by Augustus in the Roman empire, and few of the provinces proved more enthusiastically loyal to Rome. Nevertheless, the Galatians were still speaking a Celtic language in the time of Jerome (347 - 420 CE). Jerome wrote that the Galatians of Ancyra and the Treveri of Treves spoke the same language.
During his second missionary journey Paul, accompanied by Silas and Timothy (Acts 16:6), visited the "region of Galatia," where he was detained by sickness (Gal. 4:13), and had thus the longer opportunity of preaching to them the gospel. On his third journey he went over "all the country of Galatia and Phrygia in order" (Acts 18:23). During the journeys of Paul he was received with enthusiasm in Galatia.In Acts, xvi, 6 and xviii, 23:"And they went through the Phrygian and Galatian region" (ten phrygian kai Galatiken choran) and "he departed and went through the Galatian region and Phrygia" (ten Galatiken choran kai phyrgian). The Galatians were fickle; at Lystra the multitude could scarcely be restrained from sacrificing to Paul; shortly afterwards they stoned him and left him for dead. Crescens was sent thither by Paul toward the close of his life (2 Timothy 4:10).