Jason of Pherae in Wikipedia
Jason of Pherae (Ancient Greek: Ιάσων των Φερών), 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 370's. 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 to not decimate Sparta after the Battle of Leuctra. Whether or not Jason had ambitions to rule over the entire Greek peninsular- 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.
Jason however was assassinated in 370, so one can only speculate on his political and military ambitions. His son, Alexander, inherited the title of tagus and ruled harshly before finally being defeated by the Thebans.
Jason must have been an inspiration to Alexander who would follow less than half a century later. Xenophon (403-354bce) wrote of Jason: His generalship is of the highest quality-he is one who whether his methods are those of plain force, or working in the dark, or of seizing an unexpected advantage, very seldom fails to achieve his objects. He can use the night-time as well as the day time[,] and when he wants to move fast, he will put breakfast and dinner into one meal[,] so as not to interrupt his work. He will not think it right to rest until he has reached the point for which he set out and done all that had to be done. And he has trained his men to behave in the same way, although he knows how to gratify the feelings of his soldiers when they have won some success as the results of extra hard work. So all who follow him have learned this too-that one can have a good time also, if one works for it. Then, too, he is more self controlled than any man I know with regard to bodily pleasures. These never take up his time and prevent him from doing what has to be done. This easily could have been written on Alexander's tomb, as it was exactly how he lived.
The inventor of hemithorakion (half-armour equipment) is believed to be Jason of Pherae