The Byzantine Empire's history is generally periodised from late antiquity until the Fall of Constantinople in 1453 AD. From the 3rd to 6th centuries, the Greek East and Latin West of the Roman Empire gradually diverged, marked by Diocletian's (r. 284–305) formal partition of its administration in 285, the establishment of an eastern capital in Constantinople by Constantine I in 330, and the adoption of Christianity as the state religion under Theodosius I (r. 379–395), with others such as Roman polytheism being proscribed. Although the Western half of the Roman Empire had collapsed in 476, the Eastern half remained stable and emerged as one of the most powerful states in Europe, a title it held for most of its existence. Under the reign of Heraclius (r. 610–641), the Empire's military and administration were restructured and adopted Greek for official use instead of Latin. While there was an unbroken continuity in administration and other features of Roman society, historians have often distinguished the Byzantine epoch from earlier eras in Roman history for reasons including the imperial seat moving from Rome to Constantinople and the predominance of Greek instead of Latin.
The borders of the Empire evolved significantly over its existence, as it went through several cycles of decline and recovery. During the reign of Justinian I (r. 527–565), the Empire reached its greatest extent after reconquering much of the historically Roman western Mediterranean coast, including north Africa, Italy, and Rome itself, which it held for two more centuries. During the reign of Maurice (r. 582–602), the Empire's eastern frontier was expanded and the north stabilised. However, his assassination caused a two-decade-long war with Sassanid Persia which exhausted the Empire's resources and contributed to major territorial losses during the Muslim conquests of the 7th century. In a matter of years the Empire lost its richest provinces, Egypt and Syria, to the Arabs.John Haldon, Warfare, State And Society In The Byzantine World 560–1204, p.47
During the Macedonian dynasty (9th–11th centuries), the Empire again expanded and experienced a two-century long renaissance, which came to an end with the loss of much of Asia Minor to the Seljuk Turks after the Battle of Manzikert in 1071. This battle opened the way for the Turks to settle in Anatolia as a homeland. The final centuries of the Empire exhibited a general trend of decline. It struggled to recover during the 12th century, but was delivered a mortal blow during the Fourth Crusade, when Constantinople was sacked and the Empire dissolved and divided into competing Byzantine Greek and Latin realms. Despite the eventual recovery of Constantinople and re-establishment of the Empire in 1261, Byzantium remained only one of several small rival states in the area for the final two centuries of its existence. Its remaining territories were progressively annexed by the Ottomans over the 15th century. The Fall of Constantinople to the Ottoman Empire in 1453 finally ended the Roman Empire.
Tetrarchy
During the 3rd century, three crises threatened the Roman Empire: external invasions, internal civil wars and an economy riddled with weaknesses and problems.Bury (1923), 1* Fenner, Economic Factors The city of Rome gradually became less important as an administrative centre. The crisis of the 3rd century displayed the defects of the heterogeneous system of government that Augustus had established to administer his immense dominion. His successors had introduced some modifications, but events made it clearer that a new, more centralized and more uniform system was required.Bury (1923), 1
Diocletian was responsible for creating a new administrative system (the tetrarchy). He associated himself with a co-emperor, or Augustus. Each Augustus was then to adopt a young colleague, or Caesar, to share in the rule and eventually to succeed the senior partner. After the abdication of Diocletian and Maximian, however, the tetrachy collapsed, and Constantine I replaced it with the dynastic principle of hereditary succession.* Gibbon (1906), II,
Constantine I and his successors
Constantine moved the seat of the Empire, and introduced important changes into its civil and religious constitution.Gibbon (1906), III, In 330, he founded Constantinople as a second Rome on the site of Byzantium, which was well-positioned astride the trade routes between East and West; it was a superb base from which to guard the Danube river, and was reasonably close to the Eastern frontiers. Constantine also began the building of the great fortified walls, which were expanded and rebuilt in subsequent ages. J. B. Bury asserts that "the foundation of Constantinople [...] inaugurated a permanent division between the Eastern and Western, the Greek and the Latin, halves of the Empire—a division to which events had already pointed—and affected decisively the whole subsequent history of Europe."
Constantine built upon the administrative reforms introduced by Diocletian.Bury (1923), 1* Esler (2000), 1081 He stabilized the coinage (the gold solidus that he introduced became a highly prized and stable currencyEsler (2000), 1081), and made changes to the structure of the army. Under Constantine, the Empire had recovered much of its military strength and enjoyed a period of stability and prosperity. He also reconquered southern parts of Dacia, after defeating the Visigoths in 332,Carnuntum Jahrbuch 1998 page 25 and he was planning a campaign against Sassanid Persia as well. To divide administrative responsibilities, Constantine replaced the single praetorian prefect, who had traditionally exercised both military and civil functions, with regional prefects enjoying civil authority alone. In the course of the 4th century, four great sections emerged from these Constantinian beginnings, and the practice of separating civil from military authority persisted until the 7th century.Bury (1923), 25–26
Constantine the Great inaugurated the Constantine's Bridge (Danube) at Sucidava, (today Celei in Romania)Madgearu, Alexandru (2008). Istoria Militară a Daciei Post Romane 275-376. Cetatea de Scaun. , p.64 -126 in 328, in order to reconquer Dacia, a province that had been abandoned under Aurelian. He won a victory in the war and extended his control over the South Dacia, as remains of camps and fortifications in the region indicate.Barnes, Timothy D. (1981). Constantine and Eusebius. Cambridge, MA: Harvard University Press. . p 250.
Under Constantine, Christianity did not become the exclusive religion of the state, but enjoyed imperial preference, since the Emperor supported it with generous privileges: clerics were exempted from personal services and taxation, Christians were preferred for administrative posts, and bishops were entrusted with judicial responsibilities.Esler (2000), 1081* Mousourakis (2003), 327–328 Constantine established the principle that emperors should not settle questions of doctrine, but should summon general ecclesiastical councils for that purpose. The Synod of Arles was convened by Constantine, and the First Council of Nicaea showcased his claim to be head of the Church.Bury (1923), 163
The state of the Empire in 395 may be described in terms of the outcome of Constantine's work. The dynastic principle was established so firmly that the emperor who died in that year, Theodosius I, could bequeath the imperial office jointly to his sons: Arcadius in the East and Honorius in the West. Theodosius was the last emperor to rule over the full extent of the empire in both its halves.
The Eastern Empire was largely spared the difficulties faced by the West in the third and fourth centuries, due in part to a more firmly established urban culture and greater financial resources, which allowed it to placate invaders with tribute and pay foreign mercenaries. Throughout the fifth century, various invading armies overran the Western Empire but spared the east. Theodosius II further fortified the walls of Constantinople, leaving the city impervious to most attacks; the walls were not breached until 1204. To fend off the Huns of Attila, Theodosius gave them subsidies (purportedly 300 kg (700 lb) of gold).Nathan, Theodosius II (408–450 AD) Moreover, he favored merchants living in Constantinople who traded with the Huns and other foreign groups.
His successor, Marcian, refused to continue to pay this exorbitant sum. However, Attila had already diverted his attention to the Western Roman Empire.Treadgold (1995), 193 After he died in 453, his empire collapsed and Constantinople initiated a profitable relationship with the remaining Huns, who would eventually fight as mercenaries in Byzantine armies.Alemany (2000), 207* Treadgold (1997), 184
Leonid dynasty
Leo I succeeded Marcian as emperor, and after the fall of Attila, the true chief in Constantinople was the Alan general Aspar. Leo I managed to free himself from the influence of the non-Orthodox chief by supporting the rise of the Isaurians, a semi-barbarian tribe living in southern Anatolia. Aspar and his son Ardabur were murdered in a riot in 471, and henceforth, Constantinople restored Orthodox leadership for centuries.Treadgold (1997), 152–155
Leo was also the first emperor to receive the crown not from a military leader, but from the Patriarch of Constantinople, representing the ecclesiastical hierarchy. This change became permanent, and in the Middle Ages the religious characteristic of the coronation completely supplanted the old military form. In 468, Leo unsuccessfully attempted to reconquer North Africa from the Vandals.Cameron (2000), 553 By that time, the Western Roman Empire was restricted to Italy and the lands south of the Danube as far as the Balkans (the Angles and Saxons had been invading and settling Britain since the early decades of the 5th century; the Visigoths and Suebi had possessed portions of Hispania since 417, and the Vandals had entered Africa in 429; Gaul was contested by the Franks under Clovis I, Burgundians, Bretons, Visigoths and some Roman remnants; and Theodoric was destined to rule in Italy until 526).
In 466, as a condition of his Isaurian alliance, Leo married his daughter Ariadne to the Isaurian Tarasicodissa, who took the name Zeno. When Leo died in 474, Zeno and Ariadne's younger son succeeded to the throne as Leo II, with Zeno as regent. When Leo II died later that year, Zeno became emperor. The end of the Western Empire is sometimes dated to 476, early in Zeno's reign, when the Germanic Roman general Odoacer deposed the titular Western Emperor Romulus Augustulus, but declined to replace him with another puppet.
To recover Italy, Zeno could only negotiate with the Ostrogoths of Theodoric, who had settled in Moesia. He sent the gothic king to Italy as magister militum per Italiam ("commander in chief for Italy"). After the fall of Odoacer in 493, Theodoric, who had lived in Constantinople during his youth, ruled Italy on his own. Thus, by suggesting that Theodoric conquer Italy as his Ostrogothic kingdom, Zeno maintained at least a nominal supremacy in that western land while ridding the Eastern Empire of an unruly subordinate.