The hostilities were prompted by Constantine's appointment of his brother-in-law, Bassianus, as his Caesar. Bassianus was discovered to be intriguing against Constantine, perhaps at the prodding of his own brother Senecio, a close associate of Licinius. When Constantine demanded that Licinius hand over Senecio, Licinius refused. Constantine marched against Licinius, who responded by elevating another associate, Valens. The date of Valens' elevation as emperor probably occurred after the Battle of Cibalae.
Battle
The opposing armies met on the plain between the riversSava and Drave near the town of Cibalae. The battle lasted all day. The battle opened with Constantine's forces arrayed in a defile adjacent to mountain slopes. The army of Licinius was stationed on lower ground nearer the town of Cibalae, Licinius took care to secure his flanks. As the infantry of Constantine needed to move forward through broken ground the cavalry was thrown out ahead, to act as a screen. Constantine moved his formation down on to the more open ground and advanced against the awaiting Licinians Following a period of skirmishing and intense missile fire at a distance, the opposing main bodies of infantry met in close combat and fierce hand-to-hand fighting ensued. This battle of attrition was ended, late in the day, when Constantine personally led a cavalry charge from the right wing of his army. The charge was decisive, Licinius' ranks were broken. As many as 20,000 of Licinius' troops were killed in the hard-fought battle. The surviving cavalry of the defeated army accompanied Licinius when he fled the field under the cover of darkness.
Aftermath
Following the battle Licinius was forced to flee to Sirmium, and then, after collecting his family and treasury, to Thrace. Peace negotiations were initiated, but they broke down. A further battle was then fought, the Battle of Mardia, which proved to be indecisive. Heavy losses were suffered by both sides. Following the battle, in expectation of Licinius retreating on Byzantium, Constantine advanced in the direction of this city. However, Licinius had withdrawn northwards and this placed him across Constantine's lines of communication, Constantine also lost much of his baggage to Licinius. A treaty highly-favorable to Constantine was subsequently negotiated; this included the ceding by Licinius of the greater part of the Balkan Peninsula and the elevation of Constantine's sons, Crispus and Constantine, with Licinius' young son, to the rank of Caesar. Licinius then deposed and executed his erstwhile co-emperor Valens.
