The history of the Later Roman Empire covers the history of the Roman Empire from the beginning of the rule of Diocletian in 284 AD (1037 AVC) and the establishment of the Tetrarchy in 293 AD by Diocletian to the death of Heraclius in 641 AD (1394 AVC).
Background
The Roman Empire underwent a critical period of crisis after Emperor Severus Alexander was murdered in 235 AD. During the following fifty years, twenty emperors ruled, and most of them were assassinated by their own troops. In case of emergency, local officials and military commanders took full control of state administration in large regions. Examples include the Gallic Empire along the , and Queen Zenobia's Palmyrene Empire in Syria. In the east, the Persian Sasanians who replaced the Parthian Arsacids adopted an offensive policy against Rome. With the emergence of the Sasanian Empire, Rome ceased to be the sole great power in the Near East. In the west, larger tribal confederation took the place of small Germanic tribes. Among the new tribal federation, the Franks lived along the Lower Rhine, the Alemanni on the Upper Rhine, and the Goths near the Lower Danube. The Roman Empire survived the crisis with minimal territorial losses: only Dacia to the north of the Lower Danube, and the in the Black Forest region were abandoned in the 270s. Egypt and north Africa, the economically most valuable regions, were far away from the principal theatres of war, and remained almost unharmed.
The army quickly grew in size and the soldiers were no more kept away from the central territories in the 3rd century. The continued payment of soldiery could be secured only by the regular debasement of the Roman silver coins, the . As soon as the population realized that the face value of the in circulation was much higher than their silver content, inflation became uncontrollable. Old coins that contained silver or gold were quickly withdrawn from circulation and treasured. The unmanageable inflation increased the significance of taxation in kind. Regular demands for the —the compulsory grain supply to the army—and the —the mandatory military transport—put an enormous strain on the population of the highly militarized regions. The eastern cities, like Antioch and Athens, could quickly recover invasions by enemy forces, but the towns in the less prosperous western provinces were declining.
The Christians' reluctance to make sacrifices was unacceptable for most Romans. Classical authors like Tacitus and Pliny the Younger describe the Christians as opponents of traditional Roman values. Although the Christians were outlawed, they were only sporadically persecuted, and mainly fanatical extremists, especially the Phrygian Montanists were sentenced to death. Significant Christian communities existed in the largest urban centers, like Rome, Antioch, Alexandria and Carthage, and their size quickly grew in the first half of the 3rd century. Convinced that religious laxity threatened national security, Emperor Decius ordered that all citizens were to sacrifice to the gods in 249. During the ensuing persecution, great numbers of non-compliant Christians were executed or forced into exile. Decius died fighting the Goths in 251, but five years later Valerian resumed the persecution which lasted until he was captured by Sassanian troops in the Battle of Edessa in 260. Emperor Carus embarked on a new invasion of the Sassanian Empire, but a bolt of lightning struck him to death in the Persian capital, Ctesiphon in July 283. His younger son Numerian abandoned the campaign, but he died while wintering in the eastern provinces.
Tetrarchy
On learning of the death of Carus and Numerian, senior officers staying in Nicomedia proclaimed one of their number, the Illyrian Diocletian, emperor on 20 November 284. Diocletian marched to Illyricum to fight Carus' elder son, Carinus, but Carinus was assassinated by one of his own retainers in the Battle of the Margus. Diocletian, who had no son, made a Pannonian officer Maximian his co-ruler, first as Caesar in 285, then as junior Augustus in 286. The power-sharing agreement proved durable, with Diocletian mostly ruling in the East, and Maximian in the West. The diarchy developed into a tetrarchy—the rule of four co-emperors—when Diocletian appointed two officers from Illyricum, Constantius Chlorus and Galerius, as Caesars in 293. The relationship between the four emperors was reinforced through marriage alliances: Galerius married Diocletian's daughter Galeria Valeria, and Constantius wed Maximian's daughter Theodora.
Secessionist movements continued. A mutinous military commander Carausius held sway over Britain and northern Gaul from 286 until Constantius overcame him in 293. Domitius Domitianus ruled Egypt until Diocletian captured Alexandria in 297. The tetrarchs launched military campaigns along the borderlands and restored its strategic control. Galerius forced the Persian king, Narseh to cede lands along the river Tigris to Rome and reimposed Roman suzerainty over the Kingdom of Iberia. Diocletian and Galerius waged wars against the Goths, Carpi, Sarmatians, Quadi and Marcomanni along the Danube. Maximian and Constantius defeated the Franks on the Rhine. Maximian went on war against the Quinquegentiani ("Five Peoples") in Mauretania. The four co-rulers' cooperation and military achievements created a period of stability, allowing the introduction of profound administrative and financial reforms. Examples include the reorganization of the provinces and the development of a sophisticated tax system. Diocletian became convinced that the empire's integrity could only be reinforced through the renewal of the traditional religion and outlawed Christianity in 303. During the subsequent Great Persecution, many Christians suffered martyrdom.
The first tetrarchy ended with an unprecedented act, the voluntary retirement of Diocletian and Maximian on 1 May 305. On this occasion, Galerius and Constantius Chlorus were promoted to the rank of Augustus, and two Illyrian military commanders, Maximinus Daia and Valerius Severus, were appointed as the new Caesars. Their appointment apparently demonstrates Galerius' influence on the ailing Diocletian: Maximinus Daia was his nephew and Valerius Severus was his friend. Although both Constantius' son, Constantine, and Maximian's son, Maxentius, were adults, the composition of the new tetrarchy ignored their claims to succeed their fathers. When Constantius Chlorus died in Britain on 25 July 306, his troops proclaimed Constantine his successor. Three months later, Maxentius took control of southern and central Italy and Africa. Valerius Severus attacked Maxentius, but his troops who had served under Maximian mutinied and captured him. After seizing northern Italy, Maxentius persuaded his father to abandon his retirement and again rule as Augustus in spring 307. Constantine married Maximian's daughter Fausta, and his new father-in-law appointed him as Augustus. After a conflict between Maximian and Maxentius, Maximian sought refuge at Constantine, and the governor of Africa, Domitius Alexander assumed the imperial title.
To tackle the chaotic situation Galerius convinced Diocletian to preside over a conference at Carnuntum in November 308. The conference established a new tetrarchy, with Galerius and his Dacian protégé, Licinius as , and Maximinus Daia and Constantine as . Neither Maximinus nor Constantine acquiesced in their degradation, and both Maxentius and Domitius Alexander insisted on their imperial status. Maxentius sent an expeditionary force against Domitius Alexander and reconquered Africa, while Maximian staged a coup against Constantine. The coup failed and Maximian was forced to commit suicide in summer 310. After Galerius, the last surviving ruler of the first tetrarchy, died in May 311, Constantine made an alliance with Licinius against Maxentius and Maximinus Daia. Maxentius died fighting Constantine in the Battle of the Milvian Bridge on 28 October 312, and Licinius routed Maximinus Daia in Thrace on 30 April 313.
Towards Christianization
Licinius had not yet won his war against Maximinus Daia when he married Constantine's half-sister Constantia in Milan in February 313. The two emperors jointly issued a law about religious tolerance, now known as the Edict of Milan. No more excluded from imperial service, Christians could have brilliant careers, like Ablabius, a Greek of humble origin, who held the highest offices between 324 and 331. Relationship between the two emperors grew tense and Constantine seized the Dioceses of Pannonia and Moesia by force in 318. Six years later Constantine launched a new attack against Licinius and forced him to abdicate. In a year, Licinius and his about ten-year-old son by Constantia, also called Licinius, were executed. The tragic child had been appointed as Caesar along with Constantine's two eldest sons, Crispus and Constantine II in 317. Crispus' mother Minervina was Constantine's first wife, while the younger Constantine was born to Fausta. In 326, Crispus and Fausta were executed on mysterious charges, likely because of their adulterous relationship. Always hostile to the first Christian emperor, Zosimus alludes that Constantine's actual conversion to Christianity was the consequence of their execution, because only Christianity offered him absolution for his sin. Constantine made his younger sons by Fausta, Constantius II and Constans, and his nephew Dalmatius Caesars, and appointed Dalmatius' brother Hannibalianus ruler of the Pontic regions.
Constantine continued Diocletian's administrative and financial reforms, but Christian ethics had an impact on his legislation. He banned gladiator games and promoted the less violent chariot racing. He forbade the branding of slaves on the forehead, abolished penalties for celibacy, and offered financial support to poor parents to discourage infanticide. On the other hand, he prescribed that a slave nurse participating in a girl's abduction be punished with molten lead poured down her throat, and a woman who abandoned her husband was to be banished pennilessly to a remote island. Constantine established a new city at a highly defensible place on the site of the ancient Greek of Byzantium on the Bosporus in 324. In four years, his "New Rome" was surrounded by walls enclosing about of land, and it was adorned with all elements of an imperial capital, including a palace and a large stadium. The city was consecrated as Constantinople on 11 May 330. Constantine launched successful campaigns against the Goths in 332 and 336, and against the Sarmatians in 334. The new Persian king Shapur II invaded Armenia and expelled the Roman client king Tigran VII. Constantine decided to launch a counter-attack, but he died unexpectedly on 22 May 337. His stepbrothers and their sons were soon massacred likely on Constantius II's initiative. Only two children Gallus and Julian survived the purge.