Roman Empire

• July 3 – Battle of Adrianople: Emperor Constantine the Great defeats his rival Licinius near Adrianople, forcing him to retreat to Byzantium. He invades Thrace with a Visigothic force and raids the countryside.^[1]
• The Civil wars of the Tetrarchy, which began in 306, end with Constantine ruling as sole Emperor.^[1]
• July – Battle of the Hellespont: Crispus destroys Licinius' naval fleet in the Dardanelles, allowing his father Constantine the ability to cross over the Bosphorus into Asian provinces. Byzantium is besieged and Licinius assembles a second military force, under his newly elevated co-emperor Martinian at Lampsacus (modern-day Lapseki).^[1]
• September 18 – Battle of Chrysopolis: Constantine I definitively defeats Licinius at Chrysopolis, and becomes sole Emperor, thus ending the period of the Tetrarchy. Licinius escapes and gathers around 30,000 of his surviving troops at Nicomedia.^[1]
• December 19 – Licinius abdicates his position as Emperor. He is pardoned by Constantine I as a result of the supplication of his wife Constantia (who is Constantine's halfsister), and banished to Thessalonica as a private citizen.
• Constantine I founds Constantinople and incorporates Byzantium into the new capital. He reorganises the Roman army in smaller units classified into three grades: palatini, (imperial escort armies); comitatenses, (forces based in frontier provinces) and limitanei (auxilia border troops).^[2]