The accession to the purple on November 20
, of Diocletian
, the lower-class, Greek-speaking Dalmatian
commander of Carus's and Numerian's household cavalry (protectores domestici
), marked a major departure from traditional Roman constitutional theory regarding the Emperor, who was nominally first among equals during the Principate. Diocletian introduced Oriental despotism into Imperial dignity; whereas before Emperors had worn only a purple toga (toga picta
) and greeted with deference, Diocletian wore jewelled robes and shoes, and required those who greeted him to kneel and kiss the hem of his robe (adoratio
). In many ways, Diocletian was the first monarchical Emperor, and this is symbolised by the fact that the word dominus
("Lord") rapidly replaced princeps
as the favoured word for referring to the Emperor. In short, the Dominate represents a time when the emperors unabashedly showcased their status and authority compared to the earlier Principate.
The Dominate also featured a shift in the Empire's "center of gravity" from the west to the east, particularly after the establishment of Constantinople; neither Diocletian nor his co-Emperor Maximian spent much time in Rome after 286, establishing their Imperial capitals at Nicomedia and Mediolanum (modern Milan), respectively.
The Tetrarchy was a system established by Diocletian to facilitate effective government of the Empire.
After acceding to power in 284, Diocletian decide to share the load of government with his friend Maximian, whom he appointed co-regent in 286. Maximian was to use the title Caesar, while Diocletian alone was Augusti. Diocletian focused on the eastern parts of the Empire while Maximian focused on the west.
In 293, this system was further developed into the Tetrarchy: Maximian was elevated to the rank of Augustus and both Augusti appointed junior sub-emperors with the title Caesar.
There were two senior emperors (titled Augusti), one for the West and one for the East, and two junior sub-emperors (titled Caesares), one for each senior emperor. When the Augusti left office for whatever reason, the Caesares would become Augusti and appoint their own Caesares; the retired Augusti took the title senior augustus and were styled Patres Imperatorum et Caesarum ("Fathers of the Imperators and of the Caesars").
Emperors in the East
- Augustus: Diocletian, 284 – 305
- Caesar: Galerius ("Galerius Valerius Maximianus Nob. Caesar"; b. Galerius Valerius Maximianus), 293 – 305
- Augustus: Galerius ("Imp. Caesar Galerius Valerius Maximianus P.F. Invictus Aug."; b. Galerius Valerius Maximianus), 305 – 311
- Caesar: Maximinus "Daia" ("C. Valerius Galerius Maximinus Nob. Caesar"; b. C. Valerius Galerius Maximinus), 305 – 311
- Augustus: Maximinus "Daia" ("Imp. Caesar Valerius Galerius Maximinus P.F. Invictus Aug."; b. C. Valerius Galerius Maximinus), 311 – 313
Emperors in the West
- Augustus: Maximian, 293 – 305
- Note: Maximian had been co-Emperor (Caesar) with Diocletian from 286 to 293 prior to the establishment of the Tetrarchy
- Caesar: Constantius I "Chlorus" ("C. Flavius Valerius Constantius Nob. Caesar"; b. C. Flavius Valerius Constantius), 293 – 305
- Augustus: Constantius I "Chlorus" ("Imp. Caesar Flavius Valerius Constantius Aug."; b. C. Flavius Valerius Constantius), 305 – 306
- Augustus: Flavius Valerius Severus ("Imp. Caesar Severus P.F. Aug."; b. Flavius Valerius Severus), 306 – 307
- Caesar: Constantine I ("Flavius Valerius Constantinus Nob. Caesar"; b. C. Flavius Valerius Constantinus), 306 – 307
- Note: Constantius's soldiers had proclaimed Constantine augustus immediately upon Constantius's death on July 25, 306, but the augustus in the East, Galerius, acknowledged him only as caesar to Severus
- Augusti: Maxentius ("Imp. Caesar M. Aurelius Valerius Maxentius P.F. Invictus Aug."; b. M. Aurelius Valerius Maxentius) and Maximian ("Imp. Caesar M. Aurelius Valerius Maximianus P.F. Invictus Aug."; b. Maximianus), 307 – 308
- Caesar: Constantine I, 307 – 308 (styled "Imp. Caesar Constantinus P.F. Invictus Aug." from 307)
Note: In 307, the augustus Severus was murdered by mutinous soldiers while attempting to suppress the rebellion and usurpation of Maxentius, who had invited his father Maximian to return from retirement and reassume the purple as augustus with him. Maxentius and Maximian reigned in the West as augusti co-operating with Constantine as caesar until the Imperial conference at Carnutum in November 308, whereat Constantine confirmed as caesar, Maximian deposed, and Licinius appointed augustus in his place. Maxentius continued to hold power as a rival Emperor until 312; his father Maximian (the first Emperor to be restored) committed suicide after an attempt to don the purple a third time in 310.
- Augustus: Licinius ("Imp. Caesar C. Valerius Licinianus Licinius P.F. Invictus Aug."; b. C. Valerius Licinianus Licinius), 308 – 313
Diocletian's wife Prisca bore him a daughter Galeria Valeria, who married Galerius (whom Diocletian had adopted and appointed caesar on March 1, 293). Galerius's sister gave birth to a son, Maximinus Daia, and Galerius's daughter by his first wife, Valeria Maximilla, married Maxentius, son of Maximian by his wife Eutropia; Eutropia's first marriage (to Afranius Hannibalianus) had produced a daughter, Theodora, who became the second wife of Constantius Chlorus ("the Pale") in 289 (adopted by Maximian on March 1, 293). Constantius's marriage to Theodora produced a daughter, Constantia, who married Licinius; his first marriage to Helena produced a son, Constantine, whose second wife was Fausta, sister of Maxentius and daughter of Maximian.
- Diocletian: father-in-law and adoptive father of Galerius
- Maximian: father of Maxentius, adoptive father and stepfather-in-law of Constantius I "Chlorus", father-in-law of Constantine, stepgrandfather-in-law of Licinius
- Galerius: son-in-law and adopted son of Diocletian, uncle of Maximinus Daia, father-in-law of Maxentius
- Constantius I "Chlorus": father (and stepbrother-in-law) of Constantine, father-in-law of Licinius, adopted son and stepson-in-law of Maximian, adoptive brother and half-brother-in-law of Maxentius
- Maximinus Daia: nephew of Galerius
- Constantine: son (and stepbrother-in-law) of Constantius I "Chlorus", son-in-law of Maximian, brother-in-law of Maxentius, half-brother-in-law of Licinius
- Maxentius: son of Maximian, son-in-law of Galerius, adoptive brother and half-brother-in-law of Constantius I "Chlorus", brother-in-law of Constantine
- Licinius: son-in-law of Constantius I "Chlorus", half-brother-in-law of Constantine, half-nephew of Maxentius, stepgrandson-in-law of Maximian
- See also Constantinian dynasty
End of the Tetrarchy
The death of Galerius in May 311 and Constantine's spectacular victory over Maxentius at the Milvian Bridge on October 28, 312, left only three Emperors: in the East, Maximinus Daia and Licinius; in the West, Constantine. Licinius defeated Maximinus Daia in April 313 at Tarsus, and the latter committed suicide shortly thereafter, leaving Licinius and Constantine the only Emperors; they governed the Empire along the usual lines of East and West, respectively, discarded the defunct Tetrarchical system, warred against one another in 316 – 317, and again in 324 – 325. The execution of Licinius in spring 325 left Constantine the first sole Emperor since Diocletian made Maximian his co-Emperor in 286.
Emperor in the East
Emperor in the West
The Constantinian dynasty properly began with Constantius "Chlorus" (caesar, 293, augustus, 305), an experienced Illyrian soldier and general; the Constantiniani were originally another family of "Barracks Emperors". The dynasty retained and reinforced the monarchical evolution of the Imperial dignity, and sponsored the pivotal Edict of Milan in 312, which extended official toleration to Christianity, which religion had suffered considerable persecution under recent Emperors. Constantine I undertook major reforms of Imperial administration and military organisation, founded a new Imperial capital at Constantinople on November 8, 324, summoned the first Christian ecumenical council (I Nicaea, 325), and became the first Christian Emperor in 337.
Before Constantine's death, he divided the Empire into four parts governed by caesares, apparently intending to re-establish the Tetrarchy. He left most of the West to his son Constantine II, the East to his son Constantius II, Italia and the Upper Danube to his son Constans I, and Greece and the Lower Danube to his half-nephew Flavius Dalmatius. Dalmatius was killed shortly after Constantine's death, and the Empire was divided into three parts.
Emperor in Britannia, Hispania, and Gallia
- Constantine II ("Imp. Caesar Flavius Claudius Constantinus P.F. Aug."; b. Flavius Claudius Constantinus), 337 – 340
In 340, Constantine II invaded Constans I's territory in Italia; he was defeated and killed at Aquileia, and his provinces passed to the control of the brother whom he had attempted to displace.
Emperor in Italia and Africa
- Constans I ("Imp. Caesar Flavius Iulius Constans P.F. Aug."; b. Flavius Iulius Constans), 337 – 340
In 340, Constans I annexed the provinces of his late brother Constantine II, and became Emperor of the whole West.
Emperors in the West
Magnentius's defeat in 353 by Constantius II, the last of the brother Emperors, reunified the Empire under a single Emperor.
Emperor in the East
- Constantius II ("Imp. Caesar Flavius Iulius Constantius P.F. Aug."; b. Flavius Iulius Constantius), 337 – 361
In 353, Constantius II defeated the usurper Magnentius at Lyon and became sole Emperor.
Julian the Apostate famously attempted to restore paganism in the Empire, and became the second Emperor (after Decius) to die in battle with a foreign enemy (the Persians).
- Constantius II (cont'd.), 353 – 361
- Julian, 355 – 361 (as "Flavius Claudius Iulianus Nob. Caesar")
- Julian's loyalists proclaimed him Augustus in 360, but he did not fully assume the purple until Constantius II's death in November 361
- Julian ("Imp. Caesar Flavius Claudius Iulianus P.F. Aug."; b. Flavius Claudius Iulianus), 361 – 363
- Note: Julian had been co-Emperor with Constantius II from 355 until his own accession to the purple in 361
Constantius I "Chlorus" married twice; his first wife St. Helena bore him a son, Constantine I whose second wife Fausta (daughter of Maximian and Eutropia; sister of Maxentius; half-sister of Constantius I's second wife Theodora) bore him three sons (Constantine II, Constantius II, and Constans I) and two daughters (Constantia and Helena); these children were nieces and nephews of Maxentius, half-nieces and half-nephews of Licinius (who had married their father's half-sister), and grandchildren of Maximian. Constantius I's second wife Theodora (stepdaughter of Maximian and half-sister of Fausta) bore him two sons (Flavius Dalmatius and Iulius Constantius) and two daughters (Eutropia and Constantia, the wife of Licinius). Iulius Constantius's sons Constantius Gallus and Julian married Constantine I's daughters by Fausta, Constantia and Helena, respectively. Constantius II's daughter Constantia married Gratianus (see below), the son of Valentinian I (see below).
- Constantius I "Chlorus": father (and stepbrother-in-law) of Constantine I, grandfather of Constantine II, Constantius II, Constans I, and Julian the Apostate, father-in-law of Licinius, adopted son and stepson-in-law of Maximian, adoptive brother and half-brother-in-law of Maxentius
- Constantine I: son (and stepbrother-in-law) of Constantius I "Chlorus", son-in-law of Maximian, brother-in-law of Maxentius, half-brother-in-law of Licinius, father of Crispus, Constantine II, Constantius II, and Constans I, half-uncle and father-in-law of Julian the Apostate
- Constantine II: son of Constantine I, grandson of Constantius I "Chlorus", grandson of Maximian, nephew of Maxentius, half-nephew of Licinius, brother of Crispus, Constantius II, and Constans I, half-cousin and brother-in-law of Julian the Apostate
- Constantius II: son of Constantine I, grandson of Constantius I "Chlorus", grandson of Maximian, nephew of Maxentius, half-nephew of Licinius, brother of Crispus, Constantine II, and Constans I, half-cousin and brother-in-law of Julian the Apostate, father-in-law of Gratianus
- Constans I: son of Constantine I, grandson of Constantius I "Chlorus", grandson of Maximian, nephew of Maxentius, half-nephew of Licinius, brother of Crispus, Constantine II, and Constantius II, half-cousin and brother-in-law of Julian the Apostate
- Julian the Apostate: grandson of Constantius I "Chlorus", step-great-grandson of Maximian, step-great-nephew of Maxentius, half-nephew and son-in-law of Constantine I, half-cousin and brother-in-law of Constantine II, Constantius II, and Constans I
Jovian was one of Julian the Apostate's senior generals, and was chosen as his successor by the army shortly after his death in 363; he died in February 364 without heir.
- Jovian ("Imp. Caesar Flavius Iovianus P.F. Aug."; b. Flavius Iovianus), 363 – 364
The Valentinian dynasty, yet another lower-class military family (this time of Pannonian extraction), is in a very loose sense a marital continuation of the Constantinian dynasty (Gratianus was son-in-law of Constantius II, the penultimate Constantinian Emperor). Although the dynastic founder, Valentinian I, had made his career as a soldier and general, he was not a "Barracks Emperor"; rather, he was elevated to the purple by a conclave of senior generals and civil officials after the death of Jovian.
- Valentinian I ("Imp. Caesar Flavius Valentinianus P.F. Aug."; b. Flavius Valentinianus), 364 – 375
Emperors in the West
- Valentinian I, 364 – 375
- Gratian, 367 – 375 (as "Imp. Caesar Flavius Gratianus P.F. Aug.")
- Gratian ("Imp. Caesar Flavius Gratianus P.F. Aug."; b. Flavius Gratianus), 375 – 383
- Note: Gratian had been co-Emperor with Valentinian I from 367 until his own accession to the purple in 375
- Valentinian II, 375 – 383 (as "Imp. Caesar Flavius Valentinianus P.F. Aug."; b. Flavius Valentinianus)
- Valentinian II ("Imp. Caesar Flavius Valentinianus P.F. Aug."; b. Flavius Valentinianus), 383 – 392
- Note: Valentinian II had been co-Emperor with Gratian from 375 until his own accession to the purple in 383
Emperor in the East
Valens became the third Emperor (after Decius and Julian) to be killed in battle with a foreign enemy (the Goths); only two more Emperors were ever killed in battle by foreign enemies: Nikephoros I by the Bulgars in 811 and Konstantinos XI Palaeologos by the Turks in 1453.
- Valens ("Imp. Caesar Flavius Valens P.F. Aug."; b. Flavius Valens), 364 – 378
After Valens's death in 378, control of the Empire in the East passed to his nephew-in-law, Theodosius I (see below).
Valentinian I was the twice-married brother of Valens; his first wife Marina Severa bore him one son (Gratian, whose first wife was Constantia, the daughter of Constantius II), and his second wife Justina (the widow of Magnentius) bore him two children, a daughter (Galla, the second wife of Theodosius I; see below) and a son (Valentinian II).