For most of his reign, he was the colleague and rival of Constantine I, with whom he co-authored the Edict of Milan that granted official toleration to Christians in the Roman Empire.

Born to a Dacian[3][4] peasant family in Moesia Superior, Licinius accompanied his close childhood friend and future emperor Galerius, on the Persian expedition in 298.

[3] When Galerius went to deal with Maxentius personally after the death of Severus II, he left the eastern provinces in Licinius' care.

[5] Upon his return to the east Galerius elevated Licinius to the rank of Augustus in the West on 11 November 308, and under his immediate command were the Balkan provinces of Illyricum, Thrace and Pannonia.

[8] The redaction of the edict as reproduced by Lactantius – who follows the text affixed by Licinius in Nicomedia on 14 June 313, after Maximinus' defeat – uses neutral language, expressing a will to propitiate "any Divinity whatsoever in the seat of the heavens".

[7] After making the pact, Licinius rushed immediately to the East to deal with another threat, an invasion by the Persian Sassanid Empire.

[4] When he repeated this with another invasion, this time by the Goths who were pillaging Thrace under their leader Rausimod, Licinius complained that Constantine had broken the treaty between them.

Then in 324, Constantine, tempted by the "advanced age and unpopular vices"[7][5] of his colleague, again declared war against him and having defeated his army of 165,000 men[15] at the Battle of Adrianople (3 July 324), succeeded in shutting him up within the walls of Byzantium.

[citation needed] He co-authored the Edict of Milan which ended the Great Persecution, and re-affirmed the rights of Christians in his half of the empire.

[19] However, Eusebius of Caesarea, writing under the rule of Constantine, charges him with expelling Christians from the Palace and ordering military sacrifices to pagan gods, as well as interfering with the Church's internal procedures and organization.

[19] Finally, on Licinius's death, his memory was branded with infamy; his statues were thrown down; and by edict, all his laws and judicial proceedings during his reign were abolished.