Europe 317: Peace of Serdica
After his victory at Cibalae, Constantine advanced across the Balkans, narrowly defeating Licinius a second time at Mardia in early 317. However, Licinius’ continued resilience persuaded Constantine to agree to peace in return for Licinius’ cession of Europe west of Thracia to him.
? ?? 316–1 Mar 317 Valerius Valens▲
After his defeat at Cibalae, Licinius retreated to Roman Dacia, where he secured his position by appointing Valens, the Commander of the Frontier, as co-emperor. Reinforced by Valens and his forces, Licinius sent peace overtures to Constantine but was rebuffed, perhaps partly because the appointment of Valens offended Constantine. As a condition of the later Peace of Serdica (317), Licinius deposed Valens, who was executed soon after.
10 Nov 316–? ?? 321 Donatist Controversy▲
In 311 the moderate Christian Caecilian was appointed as bishop of Carthage, a move strongly opposed by the more fanatical rigorists, who instead consecrated their own bishops: first Majorinus and then, in 313, Donatus Magnus, after whom the sect would be named the Donatists. After repeated calls for compromise only exacerbated Donatist unrest and riots in Africa, Constantine ordered the confiscation of Donatist property and the exile of their leaders. The persecution peaked in 317, when imperial troops stormed a Donatist church in Carthage and massacred the congregation, but was revoked by Constantine four years later.
317 Battle of Mardia▲
In early 317 Constantine marched from Philippopolis towards Adrianople, approaching the encampment of Licinius and Valens on the plain of Mardia. The two armies engaged at dawn and fought for many hours, before a force of 5,000 that Constantine had earlier detached turned the tide of the battle by arriving on Licinius’ flank. Even so, Licinius’ troops held their ground until nightfall could aid their escape, leaving the battlefield with many thousands of dead from both sides.
1 Mar 317 Peace of Serdica▲
Following the Battle of Mardia, Constantine pushed on east, towards Byzantium, but Licinius, anticipating this move, instead moved northwest to Augusta Traiana and cut Constantine’s supply line. Rather than risk all he had achieved, Constantine accepted Licinius’ offer to a truce. In a treaty signed at Serdica (Sofia, Bulgaria) in March 317, the two Augusti agreed to resume their alliance, while Licinius acknowledged Constantine’s gains by ceding Europe west of Thraciae to him. Constantine and Licinius also named their infant sons, Constantine II and Licinius II, and Constantine’s teenage son Crispus as Caesars.