End of The Tetrarchy
The death of Galerius in May 311 and Constantine's spectacular victory over Maxentius at the Milvian Bridge on October 28, 312, left only three Emperors: in the East, Maximinus Daia and Licinius; in the West, Constantine. Licinius defeated Maximinus Daia in April 313 at Tarsus, and the latter committed suicide shortly thereafter, leaving Licinius and Constantine the only Emperors; they governed the Empire along the usual lines of East and West, respectively, discarded the defunct Tetrarchical system, warred against one another in 316 – 317, and again in 324 – 325. The execution of Licinius in spring 325 left Constantine the first sole Emperor since Diocletian made Maximian his co-Emperor in 286.
Read more about this topic: Roman Emperor (Dominate)