Licinius I was Roman Emperor from 308 to 324. Co-author of the Edict of Milan that granted official toleration to Christians in the Roman Empire, for the majority of his reign he was the rival of Constantine I. He was finally defeated at the Battle of Adrianople, before being executed on the orders of Constantine I.
Born to a Dacianpeasant family in Moesia Superior, Licinius accompanied his close childhood friend, the future emperor Galerius, on the Persian expedition in 298. He was trusted enough by Galerius that in 307 he was sent as an envoy to Maxentius in Italy to attempt to reach some agreement about his illegitimate status. Galerius then trusted the eastern provinces to him when he went to deal with Maxentius personally after the death of Flavius Valerius Severus.
Upon his return to the east, Galerius elevated Licinius to the rank of Augustus in the West on November 11, 308. He received as his immediate command the provinces of Illyricum, Thrace and Pannonia.
In 310 he took command of the war against the Sarmatians, inflicting a severe defeat on them and emerging victorious. Then on the death of Galerius, in May 311, Licinius entered into an agreement with Maximinus Daia, to share the eastern provinces between them. By this point, not only was Licinius the official Augustus of the west, but he also possessed part of the eastern provinces as well, as the Hellespont and the Bosporus became the dividing line, with Licinius taking the European provinces and Daia taking the Asian.
An alliance between Daia and Maxentius forced the two remaining emperors to enter into a formal agreement with each other. So in March 313 Licinius married Flavia Julia Constantia, half-sister of Constantine, at Mediolanum (now Milan); they had a son, Licinius the Younger, in 315. Their marriage was the occasion for the jointly-issued "Edict of Milan" that restored confiscated properties to Christian congregations and allowed Christianity to be professed in the empire.
Daia, in the meantime decided to attack Licinius. Leaving Syria with 70,000 men, he reached Bithynia, although harsh weather he encountered along the way had gravely weakened his army. In April 313, he crossed the Bosporus and went to Byzantium, which was held by Licinius' troops. Undeterred, he took the town after an eleven-day siege. He moved to Heraclea, which he captured after a short siege, before moving his forces to the first posting station. With a much smaller body of men, possibly around 30,000, Licinius arrived at Adrianople while Daia was still besieging Heraclea. On 30 April 313, the two armies clashed at the Battle of Tzirallum, and in the ensuing battle Daia's forces were crushed. Ridding himself of the imperial purple and dressing like a slave, Daia fled to Nicomedia. Believing he still had a chance to come out victorious, Daia attempted to stop the advance of Licinius at the Cilician Gates by establishing fortifications there. Unfortuately for Daia, Licinius' army succeeded in breaking through, forcing Daia to retreat to Tarsus where Licinius continued to press him on land and sea. The war between them only ended with Daia’s death in August 313.
Given that Constantine had already crushed his rival Maxentius in 312, the two men decided to divide the Roman world between them. As a result of this settlement, Licinius became sole Augustus in the East, while his brother-in-law, Constantine, was supreme in the West. Licinius immediately rushed to the east to deal with another threat, this time from the Persian Sassanids.