Celtic invasion of Greece
Celtic groups, originating frm the various La Tene chiefdoms, began a south-eastern movement into the Balkan peninsula from the fourth century BCE. Although Celtic settlements were concentrated in the western half of the Carpathian basin, there were notable incursions, and settlements, within the Balkan peninsula itself.
From their new bases in northern Illyria and Pannonia, the Gallic invasions climaxed in the early third century BCE, with the invasions of Macedonia, Thrace and Greece. The 279 BCE invasion of Greece proper was preceded by a series of other military campaigns waged toward southern Balkans and against the Macedonian Kingdom, favoured by the messy climate ensuing from the intricated succession to Alexander. A part of the invasion crossed over to Anatolia and eventually settled in the area that came to be named after them, Galatia.
Gallic invasion of the Balkans - Wikipedia