Editing List of Roman emperors (section)

== Legitimacy ==
{{See also|Roman emperor|Roman usurper}}
[[File:Denarius of Pescennius Niger.png|thumb|Coin of [[Pescennius Niger]], a [[Roman usurper]] who claimed imperial power AD 193–194. Legend: IMP CAES C PESC NIGER IVST AVG|alt=coin]]
While the imperial government of the [[Roman Empire]] was rarely called into question during its five centuries in the west and fifteen centuries in the east, individual emperors often faced unending challenges in the form of usurpation and perpetual civil wars.{{Sfn|Omissi|2018|p=3}} From the rise of [[Augustus]], the first Roman emperor, in 27 BC to the [[Sack of Rome (455)|sack of Rome]] in AD 455, there were over a hundred usurpations or attempted usurpations (an average of one usurpation or attempt about every four years). From the murder of [[Commodus]] in 192 until the fifth century, there was scarcely a single decade without succession conflicts and civil war. Very few emperors died of natural causes, with [[regicide]] in practical terms having become the expected end of a Roman emperor by [[late antiquity]].{{Sfn|Smolin|2021|p=22}} The distinction between a usurper and a legitimate emperor is a blurry one, given that a large number of emperors that were commonly considered legitimate began their rule as usurpers, revolting against the previous legitimate emperor.{{Sfn|Claes|2015|p=15}}

True legitimizing structures and theories were weak, or wholly absent in the Roman Empire,{{Sfn|Smolin|2021|p=22}} and there were no true objective legal criteria for being acclaimed emperor beyond acceptance by the [[Roman army]].{{Sfn|Omissi|2018|p=25}} Dynastic succession was not legally formalized, but also not uncommon, with powerful rulers sometimes succeeding in passing power on to their children or other relatives. While dynastic ties could bring someone to the throne, they were not a guarantee that their rule would not be challenged.{{Sfn|Claes|2015|p=23}} With the exception of [[Titus]] ({{Reign}}79–81; son of [[Vespasian]]), no son of an emperor who ruled after the death of his father died a natural death until [[Constantine the Great|Constantine I]] in 337. Control of [[Rome]] itself and approval of the [[Roman Senate]] held some importance as legitimising factors, but were mostly symbolic. Emperors who began their careers as usurpers had often been deemed [[Public enemy|public enemies]] by the senate before they managed to take the city. Emperors did not need to be acclaimed or crowned in Rome itself, as demonstrated in the [[Year of the Four Emperors]] (69), when claimants were crowned by armies in the [[Roman province]]s, and the senate's role in legitimising emperors had almost faded into insignificance by the [[Crisis of the Third Century]] (235–285). By the end of the third century, Rome's importance was mainly ideological, with several emperors and usurpers even beginning to place their court in other cities in the empire, closer to the imperial frontier.{{Sfn|Omissi|2018|pp=9, 14, 17, 24}}

Common methods used by emperors to assert claims of legitimacy, such as proclamation by the army, blood connections (sometimes fictitious) to past emperors, wearing imperial regalia, distributing one's own coins or statues and claims to pre-eminent virtue through propaganda, were pursued just as well by many usurpers as they were by legitimate emperors.{{Sfn|Smolin|2021|p=|pp=22–23}} There were no constitutional or legal distinctions that differentiated legitimate emperors and usurpers. In ancient Roman texts, the differences between emperors and "tyrants" (the term typically used for usurpers) is often a moral one (with the tyrants ascribed wicked behaviour) rather than a legal one. Typically, the actual distinction was whether the claimant had been victorious or not. In the ''[[Historia Augusta]]'', an ancient Roman collection of imperial biographies, the usurper [[Pescennius Niger]] (193–194) is expressly noted to only be a tyrant because he was defeated by [[Septimius Severus]] ({{Reign}}193–211).{{Sfn|Omissi|2018|pp=21, 29–30}} This is also followed in modern [[historiography]], where, in the absence of constitutional criteria separating them, the main factor that distinguishes usurpers from legitimate Roman emperors is their degree of success. What makes a figure who began as a usurper into a legitimate emperor is typically either that they managed to gain the recognition from a more senior, legitimate emperor, or that they managed to defeat a more senior, legitimate emperor and seize power from them by force.{{Sfn|Claes|2015|p=23}}

=== List inclusion criteria ===
Given that a concept of constitutional legitimacy was irrelevant in the Roman Empire, and emperors were only 'legitimate' in so far as they were able to be accepted in the wider empire,{{Sfn|Omissi|2018|p=34}} this list of emperors operates on a collection of inclusion criteria:

* Imperial claimants whose power across the empire became, or from the beginning was, absolute and who ruled undisputed are treated as legitimate emperors.{{Sfn|Omissi|2018|p=32}} From 286 onward, when imperial power was usually divided among two colleagues in the east and west,{{Sfn|Omissi|2018|p=xix}} control over the respective half is sufficient even if a claimant was not recognized in the other half, such as was the case for several of the last few emperors in the west.{{Sfn|Mathisen|1997}}
* Imperial claimants who were proclaimed emperors by another, legitimate, senior emperor, or who were recognized by a legitimate senior emperor, are treated as legitimate emperors.<ref>{{Harvnb|Omissi|2018|p=25}}; {{Harvnb|Claes|2015|p=18}}.</ref> Many emperors ruled alongside one or various joint-emperors. However, and specially from the 4th century onwards, most of these were children who never ruled in their own right. Scholars of the later Empire always omit these rulers,<ref>{{Harvnb|Foss|2005|p=101}}; {{Harvnb|ODB|p=360}}.</ref> but the same is not always applied during the early Empire.{{Sfnm|1a1=Elton|1y=1825|1p=303|2a1=Grant|2y=1998|2p=179|3a1=Hekster|3y=2008|3p=155|4a1=Cooley|4y=2012|5a1=Kienast|5a2=Eck|5a3=Heil|5pp=v-xii|6a1=Britannica|7a1=Livius|8a1=MET}} For the purposes of consistency, later senior emperors' tenures as junior co-emperors are not counted as part of their reign. The list also gives all co-emperors their own entry only up to the 4th century.
* Imperial claimants who achieved the recognition of the Roman Senate, especially in times of uncertainty and civil war, are, due to the senate's nominal role as an elective body, treated as legitimate emperors.<ref>{{Harvnb|Claes|2015|p=15}}; {{Harvnb|Omissi|2018|p=17}}</ref> In later times, especially when emperors ruled from other cities, this criterion defaults to the possession and control of Rome itself. In the later eastern empire, possession of the capital of [[Constantinople]] was an essential element of imperial legitimacy.{{Sfn|Van Tricht|2011|pp=79–80}}
In the case of non-dynastic emperors after or in the middle of the rule of a dynasty, it is customary among historians to group them together with the rulers of said dynasty,{{Sfn|Lawler|2004|p=323}} an approach that is followed in this list. Dynastic breaks with non-dynastic rulers are indicated with thickened horizontal lines.