Excerpts from Books and Wikipedia
"The Battle of Chrysopolis was fought on 18 September 324 at Chrysopolis (modern Üsküdar), near Chalcedon (modern Kadıköy), between the two Roman emperors Constantine I and Licinius. The battle was the final encounter between the two emperors. After his navy's defeat in the Battle of the Hellespont, Licinius withdrew his forces from the city of Byzantium across the Bosphorus to Chalcedon in Bithynia. Constantine followed, and won the subsequent battle. This left Constantine as the sole emperor, ending the period of the Tetrarchy." [Wikipedia] "Thirty years earlier, his predecessor, Diocletian, had appointed co-rulers to share the job of running the vast Roman territories—a system that had spawned multiple lines of succession. Two other men currently held parts of the empire. Licinius, a peasant who had risen through the army ranks, had claimed the title of imperator over the central part of the empire, east of the province Pannonia and west of the Black Sea; Maximinus Daia, who had also clawed his way up from peasant birth, ruled the eastern territories, which were constantly threatened by the aggressive Persian empire." [Bauer: Medieval World, p. 4]