“The kings of the east then carried on no wars without a mercenary army of Gauls; nor, if they were driven from their thrones, did they seek protection with any other people than the Gauls. Such indeed was the terror of the Gallic name, and the unvaried good fortune of their arms, that princes thought they could neither maintain their power in security, nor recover it if lost, without the assistance of Gallic valour”.
(Marcus Junianus Justinus. Epitome of the Philippic History of Pompeius Trogus XXV, 2)
Although the first Celtic mercenary activity in southeastern Europe is recorded in 367 BC, when Dionysios of Syracuse took a band of them into his service and sent them to the aid of the Macedonians against Thebes (Justin. XX, 5,6; Diod. XV, 70,1), it is not until the expansion into the Balkans and Asia-Minor at the end of the 4th / beginning of the 3rd c. BC that Celtic mercenary forces become a vital political and military factor in the Hellenistic world…