Jason of Pherae (Greek: Ἰάσων ὁ Φεραῖος) was the ruler of Thessaly during the period just before Philip II of Macedon came to power. He had succeeded his father Lycophron I of Pherae as tyrant of Pherae and was appointed tagus, or king, of Thessaly in the 370s BC and soon extended his control to much of the surrounding region. Controlling a highly trained mercenary force as well as the famous Thessalian cavalry, Jason briefly transformed Thessaly into a powerful Greek state and even spoke of invading the Persian Empire. Before writing to Philip II, Isocrates sent letters to Jason requesting that he unify Greece, as Philip later would.
The figure of Jason makes a sudden appearance in the history of classical Greece with Xenophon swiftly mentioning his name during his commentary on Theban hegemony during the 370s. From seemingly out of nowhere arose a very ambitious proto-Philip general with a hugely competent army. Xenophon quotes Jason as claiming:
‘I have men of other states as mercenaries to the number of six thousand, with whom, as I think, no city could easily contend. As for numbers,' he said, `of course as great a force might march out of some other city also; but armies made up of citizens include men who are already advanced in years and others who have not yet come to their prime. Furthermore, in every city very few men train their bodies, but among my mercenaries no one serves unless he is able to endure as severe toils as I myself’
There was a very realistic threat posed by Jason to his neighbours and arguably to all of Hellas. However, it has also been argued by Yalichev that the Thessalian showed signs of pan-Hellenism in his approach to the prominent poleis of the south, an attitude exemplified particularly in his warning to Thebes not to destroy Sparta after the Battle of Leuctra. Whether or not Jason had ambitions to rule over the entire Greek peninsula- as Philip II would after Chaeronea- can only be left to speculation. Regardless, Jason epitomises how one autocrat could suddenly rise to power through mercenary employment and threaten, both politically and militarily, his neighbouring poleis.