Theodosius I was the son of a disgraced former general of Valentinian. After Valentinian had died, his son Gratian found the empire too difficult to manage on his own. Gratian brought Theodosius out of exile and made him Emperor in the East. Theodosius avenged Gratian's death when he was toppled by the usurper Magnentius and briefly found himself to be sole emperor. He divided the empire between his two worthless sons before he died. The empire was never to be united again. Theodosius was a devoutly Christian ruler, once even doing public penance when in a fit of anger he let his soldiers pillage and loot the city of Thessalonika
For explanations of many of the abbreviations used in Roman imperial obverse legends, click here.