Roman Emperor (Dominate)


The accession on November 20, 284, of Diocletian, the lower-class, Greek-speaking Dalmatian commander of Carus's and Numerian's household cavalry, marked a major departure from traditional Roman constitutional theory regarding the Emperor, who was nominally first among equals during the Principate. Whereas before Emperors had worn only a purple toga and were greeted with deference, Diocletian wore jewelled robes and shoes, and required those who greeted him to kneel and kiss the hem of his robe. In many ways, Diocletian was the first monarchical Emperor, and this is symbolised by the fact that the word dominus rapidly replaced princeps as the favoured word for referring to the Emperor.
This period 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, respectively.

Tetrarchy

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, one for the West and one for the East, and two junior sub-emperors, 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.

Emperors in the East

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 committed suicide after an attempt to don the purple a third time in 310.
Diocletian's wife Prisca bore him a daughter Galeria Valeria, who married Galerius. 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 had produced a daughter, Theodora, who became the second wife of Constantius Chlorus in 289. 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.
To summarise:
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

The Constantinian dynasty properly began with Constantius "Chlorus", 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, and became the first Christian Emperor in 337.

Constantinian Emperors

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

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

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

In 353, Constantius II defeated the usurper Magnentius at Lyon and became sole Emperor.

Emperors

Julian the Apostate famously attempted to restore paganism in the Empire, and became the second Emperor to die in battle with a foreign enemy.
Constantius I "Chlorus" married twice; his first wife St. Helena bore him a son, Constantine the Great whose second wife Fausta bore him three sons and two daughters ; these children were nieces and nephews of Maxentius, half-nieces and half-nephews of Licinius, and grandchildren of Maximian. Constantius I's second wife Theodora bore him two sons and two daughters. 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, the son of Valentinian I.
To summarise:
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.
The Valentinian dynasty, yet another lower-class military family, is in a very loose sense a marital continuation of the Constantinian dynasty. 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 Emperors

Valens became the third Emperor to be killed in battle with a foreign enemy ; 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.
After Valens's death in 378, control of the Empire in the East passed to his nephew-in-law, Theodosius I.

Dynastic Relationships

Valentinian I was the twice-married brother of Valens; his first wife Marina Severa bore him one son, and his second wife Justina bore him two children, a daughter and a son.