Editing Tetrarchy

{{short description|Roman system of power division among four rulers}}
{{About|the tetrarchy created by Diocletian|the tetrarchy formed from the kingdom of Herod|Herodian tetrarchy|other uses|Tetrarch (disambiguation)}}
{{more footnotes needed|date=May 2012}}
{{infobox
| abovestyle = font-size:120%; color:#ffffff; background:#b80049;text-align:center;
| above = Roman imperial dynasties
| subheader = [[Tetrarchy|The Tetrarchy]]
| image = [[File:Venice – The Tetrarchs 03.jpg|200px]]
| caption = ''[[Portrait of the Four Tetrarchs]]'', two [[Porphyry (geology)|porphyry]] sculptures looted from the [[Philadelphion]] of Constantinople after 1204, now standing at the southwest corner of [[St Mark's Basilica]], [[Venice]]
| headerstyle = background:#E5E4E2; font-size: 110%;
| header1 = Chronology
| data2 = {{aligned table|fullwidth=y|leftright=y|cols=2
 | '''[[Diocletian]]''' as Augustus
 | {{nowrap|284–286}}
 | — with [[Maximian]] as Caesar
 | 285–286
 | '''[[Maximian]]''' & '''[[Diocletian]]'''<br/> as Augusti of the West and East
 | 286–305
 | — with&nbsp;[[Constantius&nbsp;I]]&nbsp;&&nbsp;[[Galerius]]<br/>as Caesares
 | 293–305
 | '''[[Constantius I]]''' & '''[[Galerius]]'''<br/>as Augusti of West and East
 | 305–306
 | — with [[Severus II]] & [[Maximinus&nbsp;II]]<br/>as Caesares
 |
 | '''[[Severus II]]''' and '''[[Galerius]]''' as Augusti of West and East
 | 306–307
 | — with [[Maximinus II]] and [[Constantine&nbsp;I]] as Caesares
 | 
 | '''[[Maxentius]]''' & '''[[Maximian]]''' as usurpers in Italy and Africa
 | 306–308
 | '''[[Galerius]]''' as Augustus
 | 307–308
 | — with [[Maximinus II]] as Caesar
 | 
 | — with [[Constantine I]] as<br/>self-proclaimed Augustus
 | 
 | '''[[Licinius]]''' & '''[[Galerius]]'''<br/>as Augusti of West and East
 | 308–311
 | — with [[Maximinus II]] and [[Constantine I]] as Caesares
 | 
 | '''[[Maxentius]]''' as usurper in Rome (and Asia Minor 311–312)
 | 308–312
 | '''[[Licinius I]]''' & '''[[Maximinus II]]'''<br/>as Augusti of West and East
 | 311–313
 | — with [[Constantine I]] as<br/>self-proclaimed Augustus
 | 
 | '''[[Constantine I]]''' & '''[[Licinius I]]'''<br/>as Augusti of West and East
 | 313–324
 | — with [[Licinius II]], [[Constantine II (emperor)|Constantine II]] & [[Crispus]] as Caesares
 | 
 | — with '''[[Valerius Valens]]''' as<br/>Augustus of the West
 | 316–317
 | — with '''[[Martinian (emperor)|Martinian]]''' as<br/>Augustus of the West
 | 324
 }}
| header3 = Succession
| data4 = {{aligned table|leftright=y|fullwidth=y
 |''Preceded by''<br/>[[Crisis of the Third Century]]
 |''Followed by''<br/>[[Constantinian dynasty]]
 }}
}}
{{Roman government}}

The '''Tetrarchy''' was the system instituted by [[Roman emperor]] [[Diocletian]] in 293 AD to govern the ancient Roman Empire by dividing it between two emperors, the ''[[augusti]]'', and their junior colleagues and designated successors, the ''[[caesares]]''. 

Initially Diocletian chose [[Maximian]] as his ''caesar'' in 285, raising him to co-''augustus'' the following year; Maximian was to govern the western provinces and Diocletian would administer the eastern ones. The role of the ''augustus'' was likened to [[Jupiter (mythology)|Jupiter]], while his ''caesar'' was akin to Jupiter's son [[Hercules]]. [[Galerius]] and [[Constantius Chlorus|Constantius]] were appointed ''caesares'' in March 293. Diocletian and Maximian retired on 1 May 305, raising Galerius and Constantius to the rank of ''augustus''. Their places as ''caesares'' were in turn taken by [[Valerius Severus]] and [[Maximinus Daza]].

The orderly system of two senior and two junior rulers endured until Constantius died in July 306, and his son [[Constantine the Great|Constantine]] was unilaterally acclaimed ''augustus'' and ''caesar'' by his father's army. Maximian's son [[Maxentius]] contested Severus' title, styled himself ''princeps invictus'', and was appointed ''caesar'' by his retired father in 306. Severus surrendered to Maximian and Maxentius in 307. Maxentius and Constantine were both recognized as ''augusti'' by Maximian that same year. Galerius appointed [[Licinius]] ''augustus'' for the west in 308 and elevated Maximinus Daza to ''augustus'' in 310.

Constantine's victory over Maxentius at the [[Battle of the Milvian Bridge]] in 312 left him in control of the western part of the empire, while Licinius was left in control of the east on the death of Maximinus Daza. Constantine and Licinius jointly recognized their sons – [[Crispus]], [[Constantine II (emperor)|Constantine II]], and [[Licinius II]] – as ''caesares'' in March 317. Ultimately the tetrarchic system lasted until c.&nbsp;324, when mutually destructive civil wars eliminated most of the claimants to power: [[Licinius]] resigned as ''augustus'' after losing the [[Battle of Chrysopolis]], leaving Constantine in control of the entire empire.

The [[Constantinian dynasty]]'s emperors retained some aspects of collegiate rule; Constantine appointed his son [[Constantius II]] as another ''caesar'' in 324, followed by [[Constans]] in 333 and his nephew [[Dalmatius]] in 335, and the three surviving sons of Constantine in 337 were declared joint ''augusti'' together, while the concept of the division of the empire under multiple joint emperors endured until the [[Fall of the Western Roman Empire]]. In the [[Eastern Roman Empire]], ''augusti'' and ''caesares'' continued to be appointed sporadically.

==Terminology==
The term ''tetrarchy'' (from the {{langx|el|τετραρχία}}, ''tetrarchia'', "leadership of four [people]"){{efn|Historian [[David Stone Potter|David Potter]] translates the term as "[[Gang of Four|gang of four]]". See idem., ''Constantine the Emperor'' (Oxford: Oxford University Press, 2013), 1.}} describes any form of government where power is divided among four individuals.

Although the term "tetrarch" was current in antiquity, it was never used in the imperial college (as it's often called) under Diocletian. Instead, the term was used to describe independent portions of a kingdom that were ruled under separate leaders.  The [[Tetrarchy (Judea)|tetrarchy of Judaea]], established after the death of [[Herod the Great]], is the most famous example of the antique tetrarchy.  The term was understood in the Latin world as well, where [[Pliny the Elder]] glossed it as follows: "each is the equivalent of a kingdom, and also part of one" (''regnorum instar singulae et in regna contribuuntur'').<ref>Qtd. and tr. Leadbetter, ''Galerius'', 3.</ref>

As used by the ancients, the term describes not only different governments, but also a different system of government from the Diocletianic arrangements.  The Judaean tetrarchy was a set of four independent and distinct states, where each tetrarch ruled a quarter of a kingdom as they saw fit; the Diocletianic tetrarchy was a [[College (Catholic canon law)|college]] led by a single supreme leader.  When later authors described the period, this is what they emphasized: [[Ammianus Marcellinus|Ammianus]] had Constantius II admonish [[Constantius Gallus|Gallus]] for disobedience by appealing to the example in submission set by Diocletian's lesser colleagues; his successor Julian compared the Diocletianic tetrarchs to a chorus surrounding a leader, speaking in unison under his command.<ref>Amm. Marc. 14.11.10; Jul. ''Caes''. 315A-B.</ref>  Only [[Lactantius]], a contemporary of Diocletian and a deep ideological opponent of the Diocletianic state, referred to the tetrarchs as a simple multiplicity of rulers.<ref>Leadbetter, ''Galerius'', 3.</ref>

Much modern scholarship was written without the term.  Although [[Edward Gibbon]] pioneered the description of the Diocletianic government as a "New Empire", he never used the term "tetrarchy"; neither did [[Theodor Mommsen]].  It did not appear in the literature until used in 1887 by schoolmaster Hermann Schiller in a two-volume handbook on the Roman Empire (''Geschichte der Römischen Kaiserzeit''), to wit: "''die diokletianische Tetrarchie''".  Even so, the term did not catch on in the literature until [[Otto Seeck]] used it in 1897.<ref>Leadbetter, ''Galerius'', 3–4.</ref>
[[File:Argenteus-Constantius I-antioch RIC 033a (obverse).jpg|thumb|On the reverse of this ''[[argenteus]]'' struck in [[Antioch]] under [[Constantius I]], the tetrarchs offer sacrifice to celebrate a victory against the [[Sarmatians]].]]

==Creation<!--Linked from 'Byzantine Empire'-->==
The first phase, sometimes referred to as the [[diarchy]] ("rule of two"), involved the designation of the general [[Maximian]] as co-emperor—firstly as ''[[Caesar (title)|caesar]]'' (heir apparent) in 285, followed by his promotion to ''augustus'' in 286. [[Diocletian]] took care of matters in the eastern regions of the empire while Maximian similarly took charge of the western regions. In 293, Diocletian thought that more focus was needed on both civic and military problems, so with Maximian's consent, he expanded the imperial college by appointing two ''caesares'' (one responsible to each ''augustus'')—[[Galerius]] and [[Constantius I]].<ref>The chronology has been thoroughly established by Kolb, ''Diocletian'', and Kuhoff, ''Diokletian''.</ref>

In 305, the senior emperors jointly abdicated and retired, allowing Constantius and Galerius to be elevated in rank to ''augustus''. They in turn appointed two new ''caesares''—[[Flavius Valerius Severus|Severus II]] in the west under Constantius, and [[Maximinus II|Maximinus]] in the east under Galerius—thereby creating the second Tetrarchy.

== Regions and capitals ==
<!--Linked from [[Roman Empire]]-->
[[File:Tetrarchy map3.jpg|thumb|left|Map of the Empire under the Tetrarchy, showing the dioceses and the four tetrarchs' zones of influence.]]

The four tetrarchs based themselves not at Rome but in other cities closer to the frontiers, mainly intended as headquarters for the defence of the empire against bordering rivals (notably [[Sassanid dynasty|Sassanian Persia]]) and [[barbarian]]s (mainly Germanic, and an unending sequence of nomadic or displaced tribes from the eastern steppes) at the [[Rhine]] and [[Danube]]. These centres are known as the tetrarchic capitals. Although Rome ceased to be an operational capital, Rome continued to be nominal capital of the entire Roman Empire, not reduced to the status of a province but under its own, unique Prefect of the city (''[[praefectus urbi]]'', later copied in Constantinople).

The four tetrarchic capitals were:

*[[Nicomedia]] in northwestern Asia Minor (modern [[İzmit]] in Turkey), a base for defence against invasion from the Balkans and Persia's Sassanids was the capital of Diocletian, the eastern (and most senior) ''augustus''; in the final reorganisation by Constantine the Great, in 318, the equivalent of his domain, facing the most redoubtable foreign enemy, [[Sassanid Persia]], became the praetorian prefecture Oriens, 'the East', the core of later Byzantium.
*[[Sirmium]] (modern [[Sremska Mitrovica]] in the [[Vojvodina]] region of modern [[Serbia]], and near [[Belgrade]], on the [[Danube]] border) was the capital of Galerius, the eastern ''caesar''; this was to become the Balkans-Danube prefecture Illyricum.
*[[Mediolanum]] (modern [[Milan]], near the Alps) was the capital of Maximian, the western ''augustus''; his domain became "Italia et Africa", with only a short exterior border.
*[[Augusta Treverorum]] (modern [[Trier]], in Germany) was the capital of Constantius, the western ''caesar'', near the strategic Rhine border; it had been the capital of Gallic emperor [[Tetricus I]]. This quarter became the prefecture Galliae.

[[Aquileia]], a port on the Adriatic coast, and [[Eboracum]] (modern [[York]], in northern England near the Celtic tribes of modern Scotland and Ireland), were also significant centres for Maximian and Constantius respectively.

In terms of regional jurisdiction there was no precise division among the four tetrarchs, and this period did not see the Roman state actually split up into four distinct sub-empires. Each emperor had his zone of influence within the Roman Empire, but little more, mainly high command in a 'war theater'. Each tetrarch was himself often in the field, while delegating most of the administration to the hierarchic bureaucracy headed by his respective [[praetorian prefect]], each supervising several [[vicarii]], the governors-general in charge of another, lasting new administrative level, the civil [[Roman diocese|diocese]]. For a listing of the provinces, now known as [[eparchy]], within each quarter (known as a praetorian prefecture), see [[Roman province]].

In the West, the ''augustus'' Maximian controlled the provinces west of the [[Adriatic Sea]] and the [[Gulf of Sidra|Syrtis]], and within that region his ''caesar'', Constantius, controlled Gaul and Britain. In the East, the arrangements between the ''augustus'' Diocletian and his ''caesar'', Galerius, were much more flexible.

==Public image==
Although power was shared in the tetrarchic system, the public image of the four members of the imperial college was carefully managed to give the appearance of a united empire (''patrimonium indivisum''). This was especially important after [[Crisis of the Third Century|the numerous civil wars of the 3rd century]].

The tetrarchs appeared identical in all official portraits. Coinage dating from the tetrarchic period depicts every emperor with identical features—only the inscriptions on the coins indicate which one of the four emperors is being shown. The Byzantine sculpture ''[[Portrait of the Four Tetrarchs]]'' shows the tetrarchs again with identical features and wearing the same military costume.

==Military successes==
One of the greatest problems facing emperors in the Third Century Crisis was that they were only ever able to personally command troops on one front at any one time. While [[Aurelian]] and [[Marcus Aurelius Probus|Probus]] were prepared to accompany their armies thousands of miles between war regions, this was not an ideal solution. Furthermore, it was risky for an emperor to delegate power in his absence to a subordinate general, who might win a victory and then be proclaimed as a rival emperor himself by his troops (which often happened). All members of the imperial college, on the other hand, were of essentially equal rank, despite two being senior emperors and two being junior; their functions and authorities were also equal.

Under the Tetrarchy a number of important military victories were secured. Both the dyarchic and the tetrarchic system ensured that an emperor was near to every crisis area to personally direct and remain in control of campaigns simultaneously on more than just one front. After suffering a defeat by the [[Sasanian Empire|Persians]] in 296, Galerius crushed [[Narseh]] in 298—reversing a series of Roman defeats throughout the century—capturing members of the imperial household and a substantial amount of booty and gaining a highly favourable peace treaty, which secured peace between the two powers for a generation. Similarly, Constantius defeated the British usurper [[Allectus]], Maximian pacified the Gauls, and Diocletian crushed the revolt of [[Domitius Domitianus|Domitianus]] in [[Egypt]].

==Demise==
{{Main|Civil wars of the Tetrarchy}}
When in 305 the 20-year term of Diocletian and Maximian ended, both abdicated. Their ''caesares'', Galerius and Constantius Chlorus, were both raised to the rank of ''augustus'', and two new ''caesares'' were appointed: [[Maximinus Daza]] (''caesar'' to Galerius) and [[Valerius Severus]] (''caesar'' to Constantius). These four formed the second tetrarchy.

However, the system broke down very quickly thereafter. When Constantius died in 306, [[Constantine the Great|Constantine]], Constantius' son, was proclaimed ''augustus'' by his father's troops; however, Galerius instead chose to promote Severus to ''augustus'' while granting Constantine the position of caesar to Severus. At the same time, [[Maxentius]], the son of Maximian, resented being left out of the new arrangements, so he rebelled against and defeated Severus before forcing him to abdicate and then arranging his murder in 307. Maxentius and Maximian both then declared themselves ''augusti''. By 308 there were therefore no fewer than four claimants to the rank of ''augustus'' (Galerius, Constantine, Maximian and Maxentius), and only one to that of ''caesar'' (Maximinus Daza).

In 308 Galerius, together with the retired emperor Diocletian and the supposedly retired Maximian, called an imperial "conference" at [[Conference of Carnuntum|Carnuntum]] on the River Danube. The council agreed that [[Licinius]] would become ''augustus'' in the West, with Constantine as his ''caesar''. In the East, Galerius remained ''augustus'' and Maximinus remained his ''caesar''. Maximian was to retire, and Maxentius was declared a usurper. This agreement proved disastrous: by 308 Maxentius had become ''de facto'' ruler of Italy and Africa even without any imperial status, and neither Constantine nor Maximinus—who had both been ''caesares'' since 306 and 305 respectively—were prepared to tolerate the promotion of the ''augustus'' Licinius as their superior.

After an abortive attempt to placate both Constantine and Maximinus with the meaningless title ''filius augusti'' ("son of the ''augustus''", essentially an alternative title for ''caesar''), they both had to be recognised as Augusti in 309. However, four full Augusti all at odds with each other did not bode well for the tetrarchic system.

Between 309 and 313 most of the claimants to the imperial office died or were killed in various civil wars. Constantine forced Maximian's suicide in 310. Galerius died naturally in 311. Maxentius was defeated by Constantine at the [[Battle of the Milvian Bridge]] in 312 and subsequently killed. Maximinus committed suicide at [[Tarsus (city)|Tarsus]] in 313 after being defeated in battle by Licinius.

By 313, therefore, there remained only two rulers: Constantine in the West and Licinius in the East. The tetrarchic system was at an end, although it took until 324 for Constantine to finally defeat Licinius, reunite the two halves of the Roman Empire and declare himself sole ''augustus''.<ref>[[Edward Gibbon|Gibbon, Edward]] (1776). "Chapter XIV". ''The History of the Decline and Fall of the Roman Empire''. Vol. II.</ref>

== Emperors ==
{| class="wikitable"
! Portrait
! Name
! Reign
! Co-ruler(s)
! Ref
|-
|[[File:Istanbul_-_Museo_archeol._-_Diocleziano_(284-305_d.C.)_-_Foto_G._Dall'Orto_28-5-2006.jpg|123x123px]]
| align="center" |[[Diocletian|'''Diocletian''' "Jovius"]]<br/><small>Gaius Aurelius Valerius Diocletianus</small>

('''''Whole, then East''''')
|20 November 284 – 1 May 305<br/><small>(20 years, 5 months and 11 days)</small>
|[[Maximian]] <small>(''caesar'', 21 July 285; co-''augustus'', 1 May 305)</small>

[[Galerius]] <small>(''caesar'', 1 March 293)</small>

[[Constantius Chlorus|Constantius I]] <small>(''caesar'', 1 March 293)</small>
|<ref name=":0">{{Cite book|last=Cooley|first=Alison E.|author-link=Alison E. Cooley |date=2012|chapter=Imperial titles, Augustus–Justinian (Appendix 2)|url=https://www.cambridge.org/core/books/cambridge-manual-of-latin-epigraphy/imperial-titles-augustusjustinian/5EA1B4FCD0E56D0F6FC9DD3903C7A7FD|access-date=2020-06-26|title=The Cambridge Manual of Latin Epigraphy|pages=488–509|chapter-url=https://books.google.com/books?id=VlghAwAAQBAJ&pg=PA488|doi=10.1017/cbo9781139020442.007|isbn=9780521840262}}</ref>
|-
|[[File:Musée_Saint-Raymond_-_2017-09-02_-_Inv._Ra_34b_-_4654.jpg|136x136px]]
| align="center" |[[Maximian|'''Maximian''' "Herculius"]]<br/><small>Marcus Aurelius Valerius Diocletianus</small>
('''''West''''')
|1 April 286 – 1 May 305<br/><small>(19 years and 1 month; retired)</small>pac

28 October 306 – 11 November 308<br/><small>(2 years and 14 days)</small>
|[[Diocletian]] <small>(''augustus'', 21 July 285; co-''augustus'', 1 May 305)</small>

[[Galerius]] <small>(''caesar'', 21 March 293)</small>

[[Constantius Chlorus|Constantius I]]  <small>(''caesar'', 1 March 293)</small>

[[Maxentius]] <small>(co-''augustus'', 306–308)</small>

[[Constantine the Great|Constantine I]] <small>(rival ''augustus'', 25 July 306; co-''augustus'', 307)</small>
|<ref name=":0" />
|-
|[[File:Romuliana_Galerius_head.jpg|137x137px]]
| align="center" |'''[[Galerius]]'''<br/><small>Gaius Galerius Valerius Maximianus</small>

('''''East''''')
|1 May 305– 5 May 311<br/><small>(6 years and 4 days)</small>
|[[Diocletian]] <small>(''augustus'', 21 March 293–1 May 305)</small>
[[Maximian]] <small>(''augustus'', 21 March 293–1 May 305)</small>

[[Constantius Chlorus|Constantius I]]  <small>(''caesar'', 1 March 293; co-''augustus'', 1 May 305–25 July 306)</small>

[[Severus II]] <small>(''caesar'', 1 May 305; co-''augustus'', August 306–April 307)</small>

[[Maxentius]] <small>(''caesar'', 28 October 306; junior co-''augustus'', April 307–May 311)</small>

[[Licinius]] <small>(designated ''augustus'' for the West, 11 November 308–311)</small>

[[Maximinus Daza|Maximinus II]] <small>(''caesar'', 1 May 305; co-''augustus'', 1 May 310–early May 311)</small>
|<ref name=":0" />
|-
|[[File:Const.chlorus01_pushkin.jpg|138x138px]]
| align="center" |[[Constantius Chlorus|'''Constantius I''' "Chlorus"]]<br/><small>Marcus Flavius Valerius Constantius</small>

('''''West''''')
|1 May 305 – 25 July 306<br/><small>(1 year, 2 months and 24 days)</small>
|[[Diocletian]] <small>(''augustus'', 1 March 293–1 May 305)</small>
[[Maximian]] <small>(''augustus'', 1 April 286–1 May 305)</small>

[[Galerius]] <small>(''caesar'', 21 March 293; co-''augustus'', 1 May 305–25 July 306)</small>

[[Severus II]] <small>(''caesar'', 1 May 305 –July 306)</small>

[[Maximinus Daza|Maximinus II]] <small>(''caesar'', 1 May 305–25 July 306)</small>
|<ref name=":0" />
|-
| align="center" |[[File:Konstantin_I._(Bronze).jpg|150x150px]]
| align="center" |[[Constantine the Great|'''Constantine I''' "the Great"]]<br/><small>Flavius Valerius Constantinus</small>

('''''West''''')
|25 July 306 – 18 September 324<br /><small>(18 years, 1 month and 25 days;<br/>sole emperor 324–337)</small>
|[[Maximian]] <small>(rival ''augustus'', 306–307; co-''augustus'', 307–308)</small>

[[Maxentius]] <small>(rival ''augustus'', 306–307; co-''augustus''; 308)</small>

[[Licinius]] <small>(rival ''augustus'', 308–310; co-''augustus'', 310–316; rival, 316–324)</small>

[[Crispus]] <small>(''caesar'', 1 March 317–324)</small>

[[Constantine II (emperor)|Constantine II]] <small>(''caesar'', 1 March 317–324)</small>

[[Martinian (emperor)|Martinian]] <small>(rival ''augustus'', 324)</small>
|<ref name=":0" />
|-
|[[File:Severus II Aureus Joanneum.jpg|100x100px]]
| align="center" |'''[[Severus II]]'''<br/><small>Flavius Valerius Severus</small>

('''''West''''')
|25 July 306 – April 307<br/><small>(8 months)</small>
|[[Galerius]] <small>(''augustus'', 25 July 306–April 307)</small>
[[Maxentius]] <small>(rival ''augustus'', 28 October 306)</small>

[[Constantine the Great|Constantine I]] <small>(rival ''augustus'', 306–307)</small>

[[Maximinus Daza|Maximinus II]] <small>(''caesar'', 1 May 305–April 307)</small>
|<ref name=":0" />
|-
|[[File:Maxentius02_pushkin.jpg|133x133px]]
| align="center" |'''[[Maxentius]]'''<br/><small>Marcus Aurelius Valerius Maxentius</small>

('''''West''''')
|28 October 306 – 28 October 312<br/><small>(6 years)</small>
|[[Maximian]] <small>(co-''augustus'', 306–308)</small>
[[Severus II]] <small>(rival ''augustus'', August 306–April 307)</small>

[[Licinius]] <small>(rival-''augustus'', 11 November 308–28 October 312)</small>

[[Constantine the Great|Constantine I]] <small>(rival ''augustus'', 25 July 306–28 October 312)</small>
|<ref name=":0" />
|-
|[[File:Bust of Licinius, Kunsthistorisches Museum.jpg|151x151px]]
| align="center" |'''[[Licinius]]'''<br/><small>Valerius Licinianus Licinius</small>

('''''West, then East''''')
|11 November 308 – 19 September 324<br/><small>(15 years, 10 months and 8 days)</small>
|[[Constantine the Great|Constantine I]] <small>(rival ''augustus'', 308–310; co-''augustus'', 310–316; rival, 316–324)</small>
[[Valerius Valens|'''Valens I''']] <small>(designated Western ''augustus'', October 316–February 317)</small>

[[Licinius II]] <small>(''caesar'', 1 March 317–324)</small>

[[Crispus]] <small>(''caesar'', 1 March 317–324)</small>

[[Constantine II (emperor)|Constantine II]] <small>(''caesar'', 1 March 317–324)</small>

[[Martinian (emperor)|'''Martinian''']] <small>(designated Western ''augustus'', July–September 324)</small>
|<ref name=":0" />
|-
|[[File:Gold Aureus of Maximinus Daia, Alexandria, AD 311 - AD 313.jpg|100x100px]]
| align="center" |[[Maximinus Daza|'''Maximinus II''' "Daza"]]<br/><small>Galerius Valerius Maximinus</small>

('''''East''''')
|310 – c. July 313<br/><small>(3 years)</small>
|[[Constantius Chlorus|Constantius I]]  <small>(''augustus'', 1 May 305–25 July 306)</small>
[[Galerius]] <small>(''augustus'', 1 May 305–25 July 311)</small>

[[Severus II]] <small>(''caesar'', 1 May 305; ''augustus'' 25 July 306)</small>

[[Maximian]] <small>(''augustus'', late 306–November 308)</small>

[[Maxentius]] <small>(rival ''augustus'', 310–312)</small>

[[Constantine the Great|Constantine I]] <small>(rival ''augustus'', 310–313)</small>

[[Licinius]] <small>(''augustus'', 308–313; rival ''augustus'', 313–313)</small>
|<ref name=":0" />
|}

==Family tree==
{{Simplified Tetrarchs family tree}}

==Detailed timeline==
{| width=50% class="wikitable" style="text-align:center"
|-
! colspan="4" |<big>Diarchy</big><br />1 April 286 – 1 March 293
|-
! colspan="2" |[[Western Roman Empire]]
! colspan="2" |[[Eastern Roman Empire]]
|-
|[[File:MSR - Tête de l'empreur Maximien Hercule - Inv 34 b (cropped).jpg|133x133px]]
| '''[[Maximian|Maximian Augustus]]'''
|[[File:Istanbul - Museo archeol. - Diocleziano (284-305 d.C.) - Foto G. Dall'Orto 28-5-2006 (cropped).jpg|100px]]
| '''[[Diocletian|Diocletian Augustus]]'''
|-
! colspan="4" |Usurpers
|-
|[[File:Aureus Carausius RIC 0005 (obverse).jpg|100px]]
| [[Carausius]]<br/>{{small|in [[Roman Britain|Britain]] (286–293)}}
| colspan="2" |—
|}
Two ''caesares'' are appointed in 293, thus starting the Tetrarchy.
{| class="wikitable" width="50%" style="text-align:center"
|-
! colspan="4" |<big>First Tetrarchy</big><br />1 March 293 – 1 May 305
|-
! colspan="2" |[[Western Roman Empire]]
! colspan="2" |[[Eastern Roman Empire]]
|-
|[[File:MSR - Tête de l'empreur Maximien Hercule - Inv 34 b (cropped).jpg|133x133px]]
| '''[[Maximian|Maximian Augustus]]'''
|[[File:Istanbul - Museo archeol. - Diocleziano (284-305 d.C.) - Foto G. Dall'Orto 28-5-2006 (cropped).jpg|100px]]
| '''[[Diocletian|Diocletian Augustus]]'''
|-
|[[File:Constantius Chlorus Ny Carlsberg Glyptotek IN836 (cropped).jpg|141x141px]]
| [[Constantius Chlorus|Constantius Caesar]]
|[[File:Romuliana Galerius head.jpg|140x140px]]
|[[Galerius|Galerius Caesar]]
|-
! colspan="4" |Usurpers
|-
|[[File:Aureus Carausius RIC 0005 (obverse).jpg|100px]]
|[[Carausius]]<br />{{small|in [[Roman Britain|Britain]] (286–293)}}
|[[File:Nummus Domitius Domitianus (obverse).jpg|105x105px]]
|[[Domitian III]]<br />{{small|in [[Roman Egypt|Egypt]] (297)}}
|-
|[[File:Aureus Allectus RIC 0001 (obverse).jpg|100x100px]]
|[[Allectus]]<br />{{small|in [[Roman Britain|Britain]] (293–296)}}
|[[File:Aschils.jpg|92x92px]]
|[[Aurelius Achilleus|Achilleus]]<br />{{small|in [[Roman Egypt|Egypt]] (297–298)}}
|}
After the retirement of the two a''ugusti'' both previous ''caesares'' succeeded them, and two new ''caesares'' were appointed.
{| class="wikitable" width="50%" style="text-align:center"
|-
! colspan="4" |<big>Second Tetrarchy</big><br />1 May 305 – 25 July 306
|-
! colspan="2" |[[Western Roman Empire]]
! colspan="2" |[[Eastern Roman Empire]]
|-
|[[File:Constantius Chlorus Ny Carlsberg Glyptotek IN836 (cropped).jpg|141x141px]]
| '''[[Constantius Chlorus|Constantius Augustus]]'''
|[[File:Romuliana Galerius head.jpg|137x137px]]
| '''[[Galerius|Galerius Augustus]]'''
|-
|[[File:Gold Aureus of Severus II (MANTIS).jpg|100x100px]]
|[[Severus II|Severus Caesar]]
|[[File:Maximinus Caesar (obverse).jpg|100x100px]]
|[[Maximinus Daza|Maximinus Caesar]]
|}
After the sudden death of [[Constantius Chlorus]] (who died of natural causes), the ''caesar'' [[Flavius Severus]] succeeded him as ''augustus''. However, Constantius' troops immediately proclaimed Constantine, Constantius' son, as their new ''augustus''. Galerius accepted Constantine as part of the imperial college, but only as ''caesar''. On 28 October 306, Maximian's son [[Maxentius]] proclaimed himself emperor in Rome. Maximian also proclaimed himself emperor, ruling jointly with his son. Despite being accepted by the [[Roman Senate]], they were not recognized by the other emperors.{{sfn|Barnes|1984|pp=30–33}}
{| class="wikitable" width="50%" style="text-align:center"
|-
! colspan="4" |<big>Third Tetrarchy</big><br />25 July 306 – September 307
|-
! colspan="2" |[[Western Roman Empire]]
! colspan="2" |[[Eastern Roman Empire]]
|-
|[[File:Severus II Aureus Joanneum.jpg|100x100px]]
| '''[[Severus II|Severus Augustus]]'''
|[[File:Romuliana Galerius head.jpg|137x137px]]
| '''[[Galerius|Galerius Augustus]]'''
|-
|[[File:Head Constantine Musei Capitolini MC1072 (cropped).jpg|133x133px]]
|[[Constantine the Great|Constantine Caesar]]<br />{{small|(self-styled ''augustus'')}}
|[[File:Maximinus Caesar (obverse).jpg|100x100px]]
|[[Maximinus Daza|Maximinus Caesar]]
|-
! colspan="4" |Usurpers
|-
| [[File:Head of Maxentius from Dresden Colosseum Rome Italy (cropped).jpg|137x137px]]
|[[Maxentius]]<br />{{small|in Italy and Africa<br/>(from 28 October 306)}}
|[[File:MSR - Tête de l'empreur Maximien Hercule - Inv 34 b (cropped).jpg|133x133px]]
|[[Maximian]]<br />{{small|in Italy and Africa<br/>(from 28 October 306)}}
|}
Severus was taken hostage by Maximian in April 307, but Galerius still acknowledged him as the official emperor of the west. Constantine was denied the promotion to ''augustus'' even after Severus' death in September, as Galerius had decided to exclude him from the system altogether. Maximian acknowledge Constantine's status as ''augustus'', but this meant nothing given that he himself was declared an usurper. Galerius and Maximinus thus remained as the only "legimitate" members of the imperial college.{{sfn|Barnes|1984|pp=30–33}}
{| class="wikitable" width="50%" style="text-align:center"
|-
! colspan="4" |<big>Galerius as sole Augustus</big><br />September 307 – November 308
|-
! colspan="2" |[[Western Roman Empire]]
! colspan="2" |[[Eastern Roman Empire]]
|-
| colspan="2" rowspan="2" |('''vacant''')
|[[File:Romuliana Galerius head.jpg|137x137px]]
| '''[[Galerius|Galerius Augustus]]'''
|-
|[[File:Maximinus Caesar (obverse).jpg|100x100px]]
|[[Maximinus Daza|Maximinus Caesar]]
|-
! colspan="4" |Usurpers
|-
| [[File:Head of Maxentius from Dresden Colosseum Rome Italy (cropped).jpg|137x137px]]
|[[Maxentius]]<br />{{small|in Italy and Africa}}
|[[File:MSR - Tête de l'empreur Maximien Hercule - Inv 34 b (cropped).jpg|133x133px]]
|[[Maximian]]<br />{{small|in Italy and Africa}}
|-
|[[File:Head Constantine Musei Capitolini MC1072 (cropped).jpg|137x137px]]
|[[Constantine the Great|Constantine]]<br />{{small|in [[Gaul]] and [[Hispania]]}}
| colspan="2" |
|}
At the council of Carnutum, Diocletian decides that Licinius will be the new ''augustus'' of the west (although his western domains only consist of the [[Diocese of Pannonia]]). Constantine was given back the title of ''caesar'', which he continued to unacknowledge.{{sfn|Barnes|1984|pp=30–33}}
{| class="wikitable" width="50%" style="text-align:center"
|-
! colspan="4" |<big>Fourth Tetrarchy</big><br />11 November 308 – May 310
|-
! colspan="2" |[[Western Roman Empire]]
! colspan="2" |[[Eastern Roman Empire]]
|-
|[[File:Bust of Licinius, Kunsthistorisches Museum (cropped).jpg|137x137px]]
| [[Licinius|'''Licinius Augustus''']]
|[[File:Romuliana Galerius head.jpg|137x137px]]
| '''[[Galerius|Galerius Augustus]]'''
|-
|[[File:Head Constantine Musei Capitolini MC1072 (cropped).jpg|137x137px]]
|[[Constantine the Great|Constantine Caesar]]<br />{{small|(self-styled ''augustus'')}}
|[[File:Maximinus Caesar (obverse).jpg|100x100px]]
|[[Maximinus Daza|Maximinus Caesar]]
|-
! colspan="4" |Usurpers
|-
| [[File:Head of Maxentius from Dresden Colosseum Rome Italy (cropped).jpg|137x137px]]
|[[Maxentius]]<br />{{small|in [[Roman Italy|Italy]]}}
|[[File:Alexander of Carthage follis (obverse).png|100x100px]]
|[[Domitius Alexander]]<br />{{small|in [[Africa (Roman province)|Africa]] (308–310?)}}
|}
Maximinus was proclaimed ''augustus'' by his troops in about May 310. Galerius reluctantly agreed to recognize both Maximinus and Constantine as ''augusti'', thus breaking the Diocletian's tetrarchic system.{{sfn|Barnes|1984|pp=30–33}}
{| class="wikitable" width="50%" style="text-align:center"
|-
! colspan="4" |<big>Tetrarchy of Augusti</big><br />May 310 – May 311
|-
! colspan="2" |[[Western Roman Empire]]
! colspan="2" |[[Eastern Roman Empire]]
|-
|[[File:Bust of Licinius, Kunsthistorisches Museum (cropped).jpg|137x137px]]
| [[Licinius|'''Licinius Augustus''']]
|[[File:Romuliana Galerius head.jpg|137x137px]]
| '''[[Galerius|Galerius Augustus]]'''
|-
|[[File:Head Constantine Musei Capitolini MC1072 (cropped).jpg|137x137px]]
|'''[[Constantine the Great|Constantine Augustus]]'''
|[[File:Gold Aureus of Maximinus Daia, Alexandria, AD 311 - AD 313.jpg|100x100px]]
|[[Maximinus Daza|'''Maximinus Augustus''']]
|-
! colspan="4" |Usurpers
|-
| [[File:Head of Maxentius from Dresden Colosseum Rome Italy (cropped).jpg|137x137px]]
|[[Maxentius]]<br />{{small|In Italy and Africa}}
|[[File:MSR - Tête de l'empreur Maximien Hercule - Inv 34 b (cropped).jpg|133x133px]]
|[[Maximian]]<br />{{small|In Gaul, {{circa}} July 310}}
|}
After the death of Galerius' (who died of natural causes), Licinius acquires parts of his domains, thus ruling over territories both in the East and West.
{| class="wikitable" width="50%" style="text-align:center"
|-
! colspan="4" |<big>Tetrarchy of Augusti</big><br />May 311 – August 313
|-
! colspan="2" |[[Western Roman Empire]]
! colspan="2" |[[Eastern Roman Empire]]
|-
|[[File:Bust of Licinius, Kunsthistorisches Museum (cropped).jpg|137x137px]]
| [[Licinius|'''Licinius Augustus''']]
| colspan="2" |('''Licinius''')
|-
|[[File:Head Constantine Musei Capitolini MC1072 (cropped).jpg|137x137px]]
|'''[[Constantine the Great|Constantine Augustus]]'''
|[[File:Gold Aureus of Maximinus Daia, Alexandria, AD 311 - AD 313.jpg|100x100px]]
|'''[[Maximinus Daza|Maximinus Augustus]]'''
|-
! colspan="4" |Usurpers
|-
| [[File:Head of Maxentius from Dresden Colosseum Rome Italy (cropped).jpg|137x137px]]
|[[Maxentius]]<br />{{small|In Italy and Africa<br/>(until 28 October 312)}}
| colspan="2" |—
|}
Licinius eventually fights and defeats Maximinus, gaining all eastern territories. He then makes peace with Constantine, who remains as the emperor of the West. This joint rule lasted until 316, when Licinius rejected Constantine's election of [[Bassianus (executed by Constantine)|Bassianus]] as ''caesar''. In the ensuing war, both ''augusti'' appointed their own sons as ''caesares'', restoring a dynastic system. Licinius appointed [[Valerius Valens|Valens]] and [[Martinian (emperor)|Martinian]] as ''augustus'' in 316 and 324 respectively (literary sources refer to them as ''caesar'', but coins bear the title ''augustus''); almost nothing is known about them.
{| class="wikitable" width="50%" style="text-align:center"
|-
! colspan="4" |<big>New Diarchy</big><br />August 313 – 18 September 324
|-
! colspan="2" |[[Western Roman Empire]]
! colspan="2" |[[Eastern Roman Empire]]
|-
|[[File:Head Constantine Musei Capitolini MC1072 (cropped).jpg|137x137px]]
| '''[[Constantine the Great|Constantine Augustus]]'''
|[[File:Bust of Licinius, Kunsthistorisches Museum (cropped).jpg|137x137px]]
|[[Licinius|'''Licinius Augustus''']]
|-
|[[File:INC-1822-a Солид Крисп цезарь ок. 317 г. (аверс).png|100x100px]]
|[[Crispus|Crispus Caesar]]<br />{{small|from 317}}
|[[File:Valerius Valens coin (transparent background).png|100x100px]]
|'''[[Valerius Valens|Valens Augustus]]'''<br />{{small|in 316}}{{efn|Nominal emperor of the West.}}
|-
|[[File:INC-2046-a Ауреус. Константин II. Ок. 337—340 гг. (аверс).png|100x100px]]
|[[Constantine II (emperor)|Constantine Caesar]]<br />{{small|from 317}}
|[[File:Martinian coin (transparent background).png|100x100px]]
|'''[[Martinian (emperor)|Martinian Augustus]]'''<br />{{small|in 324}}{{efn|Nominal emperor of the West.}}
|-
| colspan="2" |—
|[[File:Solidus Licinius Jr AF21 CdM Paris.jpg|100x100px]]
|[[Licinius II|Licinius Caesar]]<br />{{small|from 317}}
|}

== Chronological table ==
{{Chronological scheme of the Tetrarchy}}

==Legacy==
[[File:Raphael-Constantine at Milvian Bridge.jpg|thumb|200px|''Constantine at the [[battle of the Milvian Bridge]]'', fresco by [[Raphael]], Vatican Rooms.]]
Although the tetrarchic system as such only lasted until 313, many aspects of it survived. The fourfold regional division of the empire continued in the form of [[Praetorian prefecture]]s, each of which was overseen by a [[praetorian prefect]] and subdivided into administrative [[Roman diocese|dioceses]], and often reappeared in the title of the military supra-provincial command assigned to a ''[[magister militum]]''.

The pre-existing notion of ''[[consortium imperii]]'', the sharing of imperial power, and the notion that an associate to the throne was the designated successor (possibly conflicting with the notion of hereditary claim by birth or adoption), was to reappear repeatedly.

The idea of the two halves, the east and the west, re-emerged and eventually resulted in the permanent de facto division into two separate Roman empires after the death of [[Theodosius I]]; though, importantly, the Empire was never formally divided. The emperors of the eastern and western halves legally ruled as one imperial college until the [[Fall of the Western Roman Empire]] left Byzantium, the "second Rome", as the sole direct heir.

==Other examples==
*Tetrarchies in the ancient world existed in both [[Thessaly]] (in northern Greece) and [[Galatia]] (in central [[Asia Minor]]; including [[Lycaonia]]) as well as among the British [[Cantiaci]].
*The constellation of Jewish principalities in the [[Herodian kingdom]] of [[Judea]] was known as a tetrarchy; see [[Tetrarchy (Judea)]].
*In the novel ''[[The Lion, the Witch and the Wardrobe]]'', the [[Pevensie]] siblings rule [[Narnia (world)|Narnia]] as a tetrarchy of two kings and two queens.

==See also==
*{{lang|la|[[Notitia dignitatum]]}}, a later document from the imperial chancery
*{{annotated link|Problem of two emperors}}

==Notes==
{{notelist}}

==Citations==
{{reflist|30em}}

==References==
<!-- Alphabetic order-->
*{{cite book |last=Barnes |first=Timothy D. |author-link=Timothy Barnes (classicist) |url=https://archive.org/details/constantineeuseb0000barn |title=Constantine and Eusebius |publisher=Harvard University Press |year=1984 |isbn=0674165314}}
* {{cite book |last=Barnes |year=1982 |first=Timothy D. |title=The New Empire of Diocletian and Constantine |publisher=Harvard University Press |doi=10.4159/harvard.9780674280670 |place=Cambridge, MA |isbn=0-674-28066-0 |url=https://archive.org/details/barnes-new-empire}}
*{{cite book |last=Bowman |first=Alan |url=https://archive.org/details/in.ernet.dli.2015.70256 |title=The Cambridge Ancient History Volume 12, The Crisis of Empire, AD 193–337 |publisher=Cambridge University Press |year=1939 |isbn=9780521044943}}
*{{cite book
  | last = Corcoran
  | first = Simon
  | author-link=Simon Corcoran
  | url= https://books.google.com/books?id=27XwuaqMmt0C
  | title = The Empire of the Tetrarchs, Imperial Pronouncements and Government AD 284–324
  | publisher = Oxford University Press
  | year = 2000
  | isbn =019815304X }}
* {{cite book |first=Frank |last=Kolb |url=https://books.google.com/books?id=SVe6hD-MG9AC |location=Berlin |publisher=de Gruyter |language=de|title=Diocletian und die Erste Tetrarchie: Improvisation oder Experiment in der Organisation monarchischer Herrschaft? |isbn=978-3110846508 |year=2011 }}
* {{cite book |last=Kuhoff |first=Wolfgang |url=https://books.google.com/books?id=gQnFQgAACAAJ |location=Frankfurt am Main |publisher=Lang |title=Diokletian und die Epoche der Tetrarchie: Das römische Reich zwischen Krisenbewältigung und Neuaufbau (284–313 n. Chr.) |isbn=978-3631367926 |year=2001 }}
* {{cite book |last=Leadbetter |first=William Lewis |url=https://books.google.com/books?id=QBBjy7l-NWQC |location=London; New York |publisher=Routledge |title=Galerius and the Will of Diocletian |isbn=978-1135261320 |year=2009 }}
* {{Cite book|last=Rees|first=Roger|title=Diocletian and the Tetrarchy|year=2004|location=Edinburgh|publisher=Edinburgh University Press|url=https://books.google.com/books?id=wV5oAAAAMAAJ|isbn=978-0748616602}}

==External links==
{{Commons category|Tetrarchy}}
* [http://www.fourthcentury.com/index.php/evolution-tetrarchy A detailed chronology of the tetrarchy from Diocletian to Constantine]
* [http://www.fourthcentury.com/notwppages/tetrarchy-table.htm A chart showing the tetrarchy from Diocletian to Constantine]

{{Epochs of Roman Emperors}}
{{Ancient Rome topics}}

[[Category:Tetrarchy| ]]
[[Category:293 establishments]]
[[Category:313 disestablishments]]
[[Category:290s establishments in the Roman Empire]]
[[Category:310s disestablishments in the Roman Empire]]
[[Category:3rd century in the Roman Empire]]
[[Category:4th century in the Roman Empire]]
[[Category:States and territories established in the 290s]]
[[Category:States and territories disestablished in the 4th century]]
[[Category:Constitutional state types]]
[[Category:Emperors]]
[[Category:Monarchy]]
[[Category:290s in the Roman Empire]]
[[Category:Roman emperors]]
[[Category:Crisis of the Third Century]]