The success of Augustus in establishing principles of dynastic succession was limited by his outliving a number of talented potential heirs; the Julio-Claudian dynasty lasted for four more emperors—Tiberius, Caligula, Claudius, and Nero—before it yielded in AD 69 to the strife-torn Year of the Four Emperors, from which Vespasian emerged as victor. Vespasian became the founder of the brief Flavian dynasty, to be followed by the Nerva–Antonine dynasty which produced the "Five Good Emperors": Nerva, Trajan, Hadrian, Antoninus Pius and the philosophically inclined Marcus Aurelius. In the view of the Greek historian Dio Cassius, a contemporary observer, the accession of the emperor Commodus in AD 180 marked the descent "from a kingdom of gold to one of rust and iron" —a famous comment which has led some historians, notably Edward Gibbon, to take Commodus' reign as the beginning of the decline of the Roman Empire. In 212, during the reign of Caracalla, Roman citizenship was granted to all freeborn inhabitants of the Empire. Despite this gesture of universality, the Severan dynasty was tumultuous—an emperor's reign was ended routinely by his murder or execution—and following its collapse, the Roman Empire was engulfed by the Crisis of the Third Century, a period of invasions, civil strife, economic disorder, and epidemic disease. In defining historical epochs, this crisis is typically viewed as marking the start of the Later Roman Empire, and also the transition from Classical antiquity to Late antiquity. In the reign of Philip the Arab (r. 244–249), Rome celebrated the thousandth anniversary of her founding by Romulus and Remus with the Saecular Games. Diocletian (r. 284–305) restored stability to the empire, modifying the role of princeps and becoming the first emperor to be addressed by Roman citizens as domine, "master" or "lord" or referred to as dominus noster "our lord".
Diocletian's reign also brought the Empire's most concerted effort against the perceived threat of Christianity, the "Great Persecution". The state of absolute monarchy that began with Diocletian endured until the fall of the Eastern Roman Empire in 1453. Diocletian divided the empire into four regions, each ruled by an emperor (the Tetrarchy). Confident that he fixed the disorders plaguing Rome, he abdicated along with his co-augustus, and the Tetrarchy eventually collapsed in the civil wars of the Tetrarchy. Order was eventually restored by the victories of Constantine, who became the first emperor to convert to Christianity, and who founded Constantinople as a new capital for the empire after defeating his co-emperor Licinius. The reign of Julian, who under the influence of his adviser Mardonius attempted to restore Classical Roman and Hellenistic religion, only briefly interrupted the succession of Christian emperors of the Constantinian dynasty. During the decades of the Valentinianic and Theodosian dynasties, the established practice of multiple emperors was continued. Theodosius I, the last emperor to rule over both the Eastern empire and the whole Western empire, died in AD 395 after making Christianity the official religion of the Empire.