During the IVth and IInd centuries BCE, Celtic-speaking groups began to move towards and beyond already, in all likeness at least since the Late Bronze Age, Celticized mediterranean regions (Spain, southern Gaul, northern Italy).

From the IVth to the IIIrd centuries BCE, Mediterranean and Balkanic peoples “re-entered” in contact with groups of Celtic-speaking peoples Roman called “Galli” and Greeks “Galatai”, probably related to a common Celtic root *galat-i (“strong”, “strengthful”). Not that Celts were newcomers in the Mediterranean basin : not only Spain, southern Gaul and northern Italy already were partly or fully celticized since at least the Late Bronze Age, but Italian, Punic and Greek traders exchanged with them since centuries at this point; what changed is that Celtic bands or even whole peoples could be found moving toward these Mediterranean regions and beyond, especially through the dynamics of Celtic mercenariate.

While ancient Celts (i.e. mostly, but not exclusively, peoples of Gaul and southern Germania), already found a military employ even before as the Elisukoi did under Carthaginians at the Battle of Himera according Herodotos, and as some Celto-Ligues possibly did under Massaliote’s employment, the complex geopolitical situation in the hellenistic world called for the service of skilled mercenaries which made a significant part of the armies.

Ancient Celts, indeed, had put warfare in the center of their cultural, social and spiritual lives. Raiding and fighting against their neighbours was a legitimate and prestigious occupation either directly, as allies or mercenaries. Finding service elsewhere in time of peace, in lands the prestigious products they enjoyed (wine, weapons, tableware, etc.) came from was certainly particularly attractive : Polybius described these Celtic warbands as heterieiai, sort of sworn aristocratic societies, that might be identified as “männerbunds”, swnorn troops of young men anxious to prove themselves and to gather prestige and wealth among their people. But it’s quite possible these first movements of “uprooted” peoples were followed by more classically mobilized troops out of a political decision : the account made by Livy (V,3) Ambigatos, then king of Bituriges as the prime people of Gaul deciding on sending troops in Italy might be a late and incomplete record of an actual historical decision, pointing to an important tendency involving plentiful warriors, likely in groups of hundreds or thousands. This in particular might explain why some people’s names are popping up regularly in different places, as Boii or Volcae.

Following the then centuries-old trade roads, Celtic troops and warbands moved South and East, recruited in service of ambitious, powerful or needing cities or leaders, Dionysios the Elder, tyrant of Syracuse, was among the first to extensively turn to their service : one of the warbands answering the call, bullying one of the cities of Central Italy was probably at the origin of the heavily mythified story of the Sack of Rome. They were thus employed by Greeks and their enemies, finding themselves in Apulia, in central Italy, in Sicily, along the Adriatic coast, even in Greece or Africa… The wars against Carthaginians and Greeks, the rise of Rome and local rivalries led to a “golden age of mercenariate” in the region, something that did not benefited only to ancient Celts, but to many peoples of the western basin including Africans, Iberians or Italian themselves.The back-and-forthing of thousands of mercenaries, especially as they were regularly accompanied by women and likely children too, gave a migratory aspect to what was originally a matter of warfare and military service. The attractiveness of mercenariship was furthermore doubled by a general movement of population from modern southern Germany and Czechia, then inhabited by Celtic-speakers and one the centers of LaTenian archeological horizon associated with “classical” ancient Celts.The Danubian plain began to be importantly celticized, both by migrations and further contacts with indigenous peoples around the IVth century : LaTenian elements can be found as far as Ukraine, for instance, out of a mix of Celtic and Scythian populations. Likewise, Northern Gaul received an influx of Danubian newcomers, which probably through their relations with indigenous Celts, gave birth to the Belgic ensemble in northern Gaul and southern Britain.

Eventually, Romans and Greeks took the habit to refer not only to these troops as Galli/Galatai, which was possibly how they referred to themselves, but also the broad peoples they came from : thus Greeks considered northern Gaul to be inhabited by Galatai in contrast to previously known Keltoi, and Romans called northern Italy as Gallia, the land newcomers settled in important numbers (while indigenous populations were certainly already importantly Celtic-speaking).

While not necessarily related to mercenariate, especially considering climatic changes and possible worsening conditions explaining important migrations in the IVth to IIIrd centuries, they probably benefited from these regions being better known both geographically and politically, slowly expanding Celtic homeland towards potential employers or raiding targets as well allowing Galli/Galatai to refine their military techniques, giving them a certain edge, and coloring them in the eyes of Greeks as a warlike, plentiful, raiding and otherwise a nuisance although this is but one of an aspect of groups whose military and technical expertise along the intellectual sophistication of their Druids, is renowned as well.

Galli/Galatai found their way to the Adriatic coast already by the IVth century, probably due to Syracusan ambitions in the region at first, but also due to the increased Celtic presence in the Danubian plain, their chiefs even meeting Alexander in a famous episode where the young king asked them what they feared the most, certain that his reputation alone would make them answers he was, only to be answered that their only dread was the sky falling on their heads. They still not dared to wage war or raids further south, content with attacking Illyric and Thracian regions, possibly in agreement with Macedonians, forming more or less important and multi-ethnic coalitions in the region.The break-up of Alexander’s Empire provided Celts with new opportunities, accelerating and deepening LaTenian presence (possibly carrying other Germanic-speaking populations along the way, which would explain among other factors, why peoples as Basternae are considered by ancient authors as either/both Celtic or Germanic) and from there Galli/Galatai presence in the Balkans becomes a staple of IIIrd century Hellenistic military history.

By the early part of the century, raids over Macedonian and southern Thracian regions are recorded, with diverse results ranging from defeat to successful plundering and settlement (especially in Thrace), but the collapse of Lysimachus’ kingdom created a political and military vaacum that was swiftly exploited. An unusually large coalition (the “Great Expedition”) advanced south, defeating the army of Ptolemy Keraunos, himself being killed and beheaded by Gauls as a trophy/totemic standard, and splitting up in different groups, either due to disagreements or to more efficiently raid the region avoiding meeting with an unified front against them.

While relatively traumatizing, culturally and politically, the Galatian victories and the raid over Delphi was, the latter had little lasting success being repelled by either a strong resistance or epidemics and eventually moving back to Thrace where they wandered around for two years before being defeated by Antigonos Gonatas at the Battle of Lysimacheia in -278. Eventually, while most probably simply returned back to the Pannonian plain, many of them remained in Balkans.

Indeed, their defeat didn’t meant their end there : Antigonos swiftly employed defeated foes as mercenaries, as they were in the western basin of the Mediterranean region and Galatians were able to found their own kingdoms that became recruitment polls for mercenaries for all of the IIIrd century.One of them emerged around Tylis in Thrace, but other bands simply crossed the Bosphorus and the Hellespont in -278, where they entered in service of Nicomedes I.