Constantine had defeated Licinius in a previous war eight years earlier at the Battles of
Cibalae and
Campus Mardiensis. Peace was quickly arranged after this, in which Constantine conquered all of the
Balkan Peninsula, with the exception of
Thrace, and placed himself in a superior position to Licinius, leaving an unstable relationship between them. As early as 323, Constantine was ready to renew the conflict, and when his army, which was chasing an invading band of
Visigoths (or
Sarmatians), crossed the border into Licinius' territory, a timely
casus belli was present. Licinius' reaction to the trespassing was entirely hostile, which spurred Constantine to continue on the offensive. He invaded Thrace with all his strength and, although his force was smaller than that of Licinius, it was filled with battle veterans. Furthermore, since
Illyria was under his control, he had access to the best recruits in the empire. After his defeat at the
Battle of Adrianople (324), Licinius and his main army retreated to the city of
Byzantium (currently
Istanbul,
Turkey). He left a strong garrison there and crossed the
Bosphorus Strait with most of his troops. To maintain his strength in Byzantium and to secure his line of communication between the capital and his army in Asia Minor, maintaining control of the straits that separated Thrace from
Bithynia (Bosphorus) and
Mysia (Hellespont) became imperative for Licinius. To cross into Asia to overcome Licinius' resistance, Constantine would also have to gain maritime control of the straits. Licinius' main army was on the Bosphorus to guard it while the majority of his navy moved to cover the Hellespont. He also assembled a second force under his newly elevated co-emperor
Martinian (r. 324) at
Lampsacus (present-day
Lapseki) on the Asiatic coast of the Hellespont. == Siege and aftermath ==