Title: Middle Eastern empires
Content:   Constantinople, situated on the Bosporus Straits at the mouth of the Black Sea, became the capital of the Roman Empire in 330 CE after Constantine the Great, the first Christian emperor, refounded the city of Byzantium. Although the city was called Constantinople until its fall, the Eastern Roman Empire became known by the classical name of Byzantium, and often the city was called by its old name as well. The city's status as residents of the Eastern Roman Emperor made it into the premier city in all of the Eastern Roman colonies in the Balkans, Syria, Jordan, Israel, Lebanon, Cyprus, Egypt, 
Title: History of the Roman Empire
Content:  Arcadius became ruler in the East, with his capital in Constantinople, and Honorius became ruler in the West, with his capital in Milan and later Ravenna. The Roman state would continue to have two different emperors with different seats of power throughout the 5th century, though the eastern Romans considered themselves to be the only ones who were fully Roman. Latin was used in official writings as much as, if not more than, Greek and the two halves were nominally, culturally and historically, if not politically, the same state. Arcadius died in 408, having already elevated his infant son Theodosius 
Title: Constantine the Great
Content:  capital should represent the integration of the East into the Roman Empire as a whole, as a center of learning, prosperity, and cultural preservation for the whole of the Eastern Roman Empire. Among the various locations proposed for this alternative capital, Constantine appears to have toyed earlier with Serdica (present-day Sofia), as he was reported saying that "Serdica is my Rome". Sirmium and Thessalonica were also considered. Eventually, however, Constantine decided to work on the Greek city of Byzantium, which offered the advantage of having already been extensively rebuilt on Roman patterns of urbanism, during the preceding century, by Septimius Severus and Caracalla, who had already acknowledged its strategic importance. The city was thus founded in 324, dedicated on 11 May 330 and renamed Constantinopolis ("Constantine's City" or Constantinople in English). Special 
Title: Emperor
Content:   Historians generally refer to the continuing Roman Empire in the east as the Byzantine Empire after Byzantium, the original name of the town that Constantine I would elevate to the Imperial capital as New Rome in AD 330. (The city is more commonly called Constantinople and is today named Istanbul). Although the empire was again subdivided and a co-emperor sent to Italy at the end of the fourth century, the office became unitary again only 95 years later at the request of the Roman Senate and following the death of Julius Nepos, last Western Emperor. This change was a recognition of the reality that little remained of Imperial authority in the areas 
Title: Roman Italy
Content:  It finally gained the rank of eastern capital when given an Praefectus Urbi in 359 and the senators who were clari became senators of the lowest rank as clarissimi. As a result, Italy began to decline in favour of the provinces, which resulted in the division of the Empire into two administrative units in 395: the Western Roman Empire, with its capital at Mediolanum (now Milan), and the Eastern Roman Empire, with its capital at Constantinople (now Istanbul). In 402, the Imperial residence was moved to Ravenna from Milan, confirming the decline of the city of Rome (which was sacked in 410 for the first time in almost eight centuries).
Title: Geopolitics of the Roman Empire
Content:  Nicomedia as his capital due to its more central location between Europe and Asia and it became the Empire's most senior city. When Constantine I became sole Emperor he built a new capital at Constantinople because of its site with easy access to the Danube and Euphrates frontiers able to meet the European barbarian and Sassanid threat respectively as well as its more defensible location than Nicomedia, surrounded as it was with water on three sides. After the battle of Adrianople, the Rhine and Danube frontiers became incredibly porous and an independent horde of Goths were loose within the borders of the empire. Alaric I's siege of Milan in 403 was enough to persuade emperor Honorius to move the capital to a less exposed location and so Ravenna became capital of the Western Empire as it was surrounded by swamps on all sides and harder to lay siege to.
Title: Historiography of the fall of the Western Roman Empire
Content:  longer served as an effective capital for the Emperor and various cities were used as new administrative capitals. Successive emperors, starting with Constantine, privileged the eastern city of Byzantium, which he had entirely rebuilt after a siege. Later renamed Constantinople, and protected by formidable walls in the late 4th and early 5th centuries, it was to become the largest and most powerful city of Christian Europe in the Early Middle Ages. Since the Crisis of the Third Century, the Empire was intermittently ruled by more than one emperor at once (usually two), presiding over different regions. At first a haphazard form 
Title: Istanbul
Content:    Constantine the Great effectively became the emperor of the whole of the Roman Empire in September 324. Two months later, he laid out the plans for a new, Christian city to replace Byzantium. As the eastern capital of the empire, the city was named Nova Roma; most called it Constantinople, a name that persisted into the 20th century. On 11 May 330, Constantinople was proclaimed the capital of the Roman Empire, which was later permanently divided between the two sons of Theodosius I upon his death on 17 January 395, when the city became the capital of the Eastern Roman (Byzantine) Empire. The establishment of Constantinople was one 
Title: Dominate
Content:  finally replaced Nicomedia as the site of the emperor's residence in the east in 330. Constantinople would remain the capital of the eastern provinces throughout the period of the Dominate. In the west, Mediolanum continued to be the imperial residence until the repeated invasions by Alaric I forced the western emperor Honorius to relocate to the strongly fortified city of Ravenna in 402. Ravenna remained the western imperial capital until the loss of Italy in 476. Although Rome was reincorporated into the empire by Justinian I in 540, it was Ravenna which was selected as the official residence of the Exarch, the governor who represented the emperor in Italy. Ravenna would retain this position until 751, when the Byzantine Empire finally lost the central Italian provinces to the Lombards.
Title: History of Western civilization before AD 500
Content:  for urban dwellers). The population and wealth of the Roman Empire had been shifting east, and the division of Europe into a Western (Latin) and an Eastern (Greek) part was prefigured in the division of the Empire by the Emperor Diocletian in AD 285. Around 330, Constantine established the city of Constantinople as a new imperial city which would be the capital of the Byzantine Empire. Possessed of mighty fortifications and architectural splendour, the city would stand for another thousand years as a "Roman Capital". The Hagia Sophia Cathedral (later converted to a mosque following the Fall of Constantinople in 1453) is one of 

Question: what was the capital city of the east roman empire?
Answer: Constantinople.