Tetrarchy

The Tetrarchy was the system instituted by Roman Emperor Diocletian in 293 to govern the ancient Roman Empire by dividing it between two senior emperors, the "augusti", and their juniors and designated successors, the "caesares". This marked the end of the Crisis of the Third Century.
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, while his "caesar" was akin to Jupiter's son Hercules. Galerius and 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 emperors endured until Constantius died in July 306, and his son Constantine was unilaterally acclaimed "augustus" and "caesar" by his father's army. Maximian's son Maxentius contested Severus's 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, and Licinius II – as "caesares" in March 317. Ultimately the tetrarchic system lasted until c. 324, when mutually destructive civil wars eliminated most of the claimants to power: Licinius resigned as "augustus" after the 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, and 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 , "tetrarchia", "leadership of four [people]") describes any form of government where power is divided among four individuals.
Although the term "tetrarch" was current in antiquity, it was never used of the imperial college under Diocletian. Instead, the term was used to describe independent portions of a kingdom that were ruled under separate leaders. The 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").
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 led by a single supreme leader. When later authors described the period, this is what they emphasized: Ammianus had Constantius II admonish 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. 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.
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.
Creation.
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" (junior emperor) 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.
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"—Severus II in the west under Constantius, and Maximinus in the east under Galerius—thereby creating the second Tetrarchy.
Regions and capitals.
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 Sassanian Persia) and barbarians (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:
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 Pretorian Prefect, each supervising several Vicarii, the governors-general in charge of another, lasting new administrative level, the civil 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 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.
However, it appears that some contemporary and later writers, such as the Christian author Lactantius, and Sextus Aurelius Victor (who wrote about fifty years later and from uncertain sources), misunderstood the tetrarchic system in this respect, believing it to have involved a stricter division of territories among the four emperors.
Public image.
Although power was shared in the tetrarchic system, the public image of the four emperors in the imperial college was carefully managed to give the appearance of a united empire ("patrimonium indivisum"). This was especially important after 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 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 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 Domitianus in Egypt.
Demise.
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, Galerius promoted Severus to "augustus" while Constantine, Constantius' son, was proclaimed "augustus" by his father's troops. At the same time, Maxentius, the son of Maximian, who also resented being left out of the new arrangements, 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 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 rank, 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 in 313 after being defeated in battle by Licinius.
By 313, therefore, there remained only two emperors: 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".
Simplified timeline.
Tetrarchy until July 306.
After the retirement of the two "Augusti" both previous 
"caesares" succeeded them, and two new "caesares" were appointed. Maximinus Daia was Galerius' nephew.
Tetrarchy until 16 May 307.
After the death of Constantius his legions proclaim his son Constantine the new "augustus", but Galerius elevates Severus to be the new "junior augustus" and compensates Constantine with the rank of "caesar".
Tetrarchy from 18 November 308 to the beginning of May 311.
After the death of Severus, Constantine does not succeed him. At the council of Carnutum, Diocletian decides that 
Licinius will be the new "augustus" of the West.
Tetrarchy from May 311.
After the death of Galerius he was succeeded by Maximinus Daza in as "augustus" of the East, but is crowded by Licinius, who wants to have the status of the "senior augustus". Maximinus appoints no new "caesar", although it was assumed that this position should later on be filled out with the son of Severus, Flavius Severianus, or at least that he was scheduled for this position.
Tetrarchy after 8 October 316 to the end of 316.
Shortly before the turn of the year 316/317, Constantine, now "augustus" in the West, appointed a "caesar", while Licinius briefly appointed one of his officers, Valerius Valens, as the third "augustus". This was apparent from coins, though Valens was apparently inferior to Licinius, who soon executed him. Even the chronology is unclear, as the date stamping could also be the turn of the year 314/315.
Tetrarchy from 1 March 317 to 18 September 324.
The tetrarchic system is at its end. Both "Augusti" appoint their own sons as co-emperors, restoring a dynastic system. However, before his death, Licinius appoints the General Martinianus on 3 July 324 as "augustus" in name only, as Martinianus was intended to replace Constantine in the west.
Legacy.
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 prefectures, each of which was overseen by a praetorian prefect and subdivided into administrative 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 it is important to remember that the empire was never formally divided, the emperors of the eastern and western halves legally ruling as one imperial college until the fall of Rome's western empire left Byzantium, the "second Rome", sole direct heir.