Final answer:
After the eight-year civil war, Diocletian's successor was Constantine.
Step-by-step explanation:
After Diocletian's eight-year civil war and his retirement in 305 CE, his co-emperor Maximian also left office. The Tetrarchy, which was designed to ensure a smooth transition of power, faltered as the remaining two tetrarchs were replaced by new junior emperors, leading to a new round of civil wars.
Constantine, the son of Tetrarch Constantius, rose to power through a military campaign and claimed sole control of the Empire in 312 CE, appointing no co-emperor.