On an uncertain date in the spring of 325, the Roman Emperor Constantine the Great had his onetime co-emperor — and now prisoner — Licinius executed for a purportedly treasonable plot.
In the system of tetrarchy whereby the Roman world was divided in two, each half governed by an Augustus with a lieutenant Caesar, Constantine and Licinius had established themselves as masters of the west and the east, respectively.
History, which records Constantine as the vessel of Christianity’s political triumph, recommends religious faction as the cause of the strife between them: the two had jointly promulgated the Edict of Milan establishing religious toleration, but their realms had become poles of the two hostile religions — rising Christendom gathering under Constantine’s banner; the pagan world it would supplant dominant under Licinius. The latter is said to have reneged his toleration, though not necessarily to the extent of a full persecution.
Whether we can accept religious policy as a cause sufficient to throw the Roman world into civil war, or suspect more prosaic rivalries over land and power, the two were at one another’s throats before long. Conflicts, invariably won by Constantine, and truces stabilizing an increasingly one-sided balance of power, punctuated the fraying relationship during the decade before Licinius’ decisive defeat.
Upon his deposition, Licinius was allowed to live, courtesy of the offices of his wife, Constantine’s half-sister — legacy of bygone imperial marital politics — but his confinement in Thessalonica didn’t last long.
Fifth-century Greek historian Socrates Scholasticus describes the former emperor’s allegedly treasonable end:
Accordingly he having taken him alive, treated him with the utmost humanity, and would by no means put him to death, but ordered him to take up his abode and live in tranquillity at Thessalonica. He having, however, remained quiet a short time, managed afterwards to collect some barbarian mercenaries and made an effort to repair his late disaster by a fresh appeal to arms. The emperor being made acquainted with his proceedings, directed that he should be slain, which was carried into effect. Constantine thus became possessed of the sole dominion, and was accordingly proclaimed sovereign Autocrat.
Writing much later, Gibbon had a more skeptical interpretation of this convenient execution:
[Licinius’] confinement was soon terminated by death, and it is doubtful whether a tumult of the soldiers, or a decree of the senate, was suggested as the motive for his execution. According to the rules of tyranny, he was accused of forming a conspiracy, and of holding a treasonable correspondence with the barbarians; but as he was never convicted, either by his own conduct or by any legal evidence, we may perhaps be allowed, from his weakness, to presume his innocence. The memory of Licinius was branded with infamy, his statues were thrown down, and by a hasty edict, of such mischievous tendency that it was almost immediately corrected, all his laws, and all the judicial proceedings of his reign, were at once abolished. By this victory of Constantine, the Roman world was again united under the authority of one emperor, thirty-seven years after Diocletian had divided his power and provinces with his associate Maximian.
It was upon Licinius’ tomb that Constantine built that legacy so fundamental to the western world down to the present day. That same pregnant year of 325, he would summon the Council of Nicea, establishing Christian orthodoxy in a pact with temporal power; soon after, he built up Constantinople, to which he then relocated his court and transferred to the future Byzantine Empire such brio as still persisted in the listing hulk of Rome.