Roman Empire

===Transition from Classical to Late Antiquity===
{{Main|Later Roman Empire|Fall of the Western Roman Empire}}
{{See also|Barbarian kingdoms|Byzantine Empire}}
[[File:Invasions of the Roman Empire 1.png|upright=1.35|thumb|The [[Migration Period|Barbarian Invasions]] consisted of the movement of (mainly) ancient [[Germanic peoples]] into Roman territory. Historically, this event marked the transition between [[classical antiquity]] and the [[Middle Ages]].]]

In the view of contemporary Greek historian [[Cassius Dio]], the accession of [[Commodus]] in 180 marked the descent "from a kingdom of gold to one of rust and iron",<ref>{{Citation |last=[[Dio Cassius]] |title=Roman History |edition=Loeb Classical Library edition, 1927 |translator-last=Cary |translator-first=E. |page=[https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Cassius_Dio/72*.html#36 72.36.4]}}</ref> a comment which has led some historians, notably [[Edward Gibbon]], to take Commodus' reign as the beginning of the [[Historiography of the fall of the Western Roman Empire|Empire's decline]].<ref name="Commodus-Gibbon">{{Citation |last=Gibbon |first=Edward |title=The History of the Decline And Fall of the Roman Empire |date=1776 |chapter=The Decline And Fall in the West – Chapter 4 |chapter-url=https://www.ccel.org/g/gibbon/decline/volume1/chap4.htm |author-link=Edward Gibbon |access-date=27 June 2017 |archive-date=24 August 2017 |archive-url=https://web.archive.org/web/20170824100850/http://www.ccel.org/g/gibbon/decline/volume1/chap4.htm |url-status=live }}</ref>{{Sfnp|Goldsworthy|2009|p=50}}

In 212, during the reign of [[Caracalla]], [[Roman citizenship]] was granted to all freeborn inhabitants of the empire. The [[Severan dynasty]] was tumultuous; an emperor's reign was ended routinely by his murder or execution and, following its collapse, the Empire was engulfed by the [[Crisis of the Third Century]], a period of [[invasion]]s, [[civil strife]], [[Economic collapse|economic disorder]], and [[Plague of Cyprian|plague]].<ref>{{Cite book |last=Brown |first=Peter |title=The World of Late Antiquity |date=1971 |publisher=Harcourt Brace Jovanovich |isbn=978-0-151-98885-3 |page=22 |author-link=Peter Brown (historian)}}</ref> In defining [[periodization|historical epochs]], this crisis sometimes marks the transition from [[Classical Antiquity|Classical]] to [[Late Antiquity]]. [[Aurelian]] ({{R.|270|275}}) stabilised the empire militarily and [[Diocletian]] reorganised and restored much of it in 285.{{Sfnp|Goldsworthy|2009|pp=405–415}} Diocletian's reign brought the empire's most concerted effort against the perceived threat of [[early Christianity|Christianity]], the "[[Diocletianic Persecution|Great Persecution]]".{{Citation needed|date=February 2024}}

Diocletian divided the empire into four regions, each ruled by a separate [[Tetrarchy|tetrarch]].<ref>{{Cite book |last=Potter |first=David |title=The Roman Empire at Bay |date=2004 |publisher=Routledge |isbn=978-0-415-10057-1 |pages=296–298 |author-link=David Stone Potter}}</ref> Confident that he fixed the disorder plaguing Rome, he abdicated along with his co-emperor, but the Tetrarchy [[Civil wars of the Tetrarchy|collapsed shortly after]]. Order was eventually restored by [[Constantine the Great]], who became the first emperor to [[Constantine the Great and Christianity|convert to Christianity]], and who established [[Constantinople]] as the new capital of the Eastern Empire. During the decades of the [[Constantinian dynasty|Constantinian]] and [[Valentinian dynasty|Valentinian]] dynasties, the empire was divided along an east–west axis, with dual power centres in Constantinople and Rome. [[Julian (emperor)|Julian]], who under the influence of his adviser [[Mardonius (philosopher)|Mardonius]] attempted to restore [[Religion in ancient Rome|Classical Roman]] and [[Hellenistic religion]], only briefly interrupted the succession of Christian emperors. [[Theodosius I]], the last emperor to rule over both East and West, died in 395 after making Christianity the [[State church of the Roman Empire|state religion]].<ref>{{Cite book |last=Starr |first=Chester G. |title=A History of the Ancient World |date=1974 |publisher=Oxford University Press |isbn=978-0-195-01814-1 |edition=2nd |pages=670–678 |author-link=Chester G. Starr |orig-date=1965}}</ref>
[[File:628px-Western and Eastern Roman Empires 476AD(3).PNG|thumb|The Roman Empire by 476, noting western and eastern divisions]] [[File:The_Roman_Empire,_AD_395.png|right|thumb|upright=1.15|The administrative divisions of the Roman Empire in 395 AD]]