This volume covers the history of the Roman Empire from the accession of Septimius Severus in AD 193 to the death of Constantine in AD 337. This period was one of the most critical in the history of the Mediterranean world. It begins with the establishment of the Severan dynasty as a result of civil war. From AD 235 this period of relative stability was followed by half a century of short reigns of short-lived emperors and a number of military attacks on the eastern and northern frontiers of the empire. This was followed by the First Tetrarchy (AD 284-305), a period of collegial rule in which Diocletian, with his colleague Maximian and two junior Caesars (Constantius and Galerius), restabilised the empire. The period ends with the reign of the first Christian emperor, Constantine, who defeated Licinius and established a dynasty which lasted for thirty-five years.