Empire, southeastern and southern Europe and western Asia. It began as the city of Byzantium, which had grown from an ancient Greek colony founded on the European side of the Bosporus. The city was taken in AD 330 by Constantine I, who refounded it as Constantinople. The area at this time was generally termed the Eastern Roman Empire. The fall of Rome in 476 ended the western half of the Roman Empire; the eastern half continued as the Byzantine Empire, with Constantinople as its capital. The eastern realm differed from the west in many respects: heir to the civilization of the Hellenistic era, it was more commercial and more urban. Its greatest emperor, Justinian (r. 527–565), reconquered some of western Europe, built the Hagia Sophia, and issued the basic codification of Roman law. After his death the empire weakened. Though its rulers continued to style themselves “Roman” long after Justinian's death, “Byzantine” more accurately describes the medieval empire. The long controversy over iconoclasm within the eastern church prepared it for the break with the Roman church (see Schism of 1054). During the controversy, Arabs and Seljuq Turks increased their power in the area. In the late 11th century, Alexius I Comnenus sought help from Venice and the pope; these allies turned the ensuing Crusades into plundering expeditions. In the Fourth Crusade the Venetians took over Constantinople and established a line of Latin emperors. Recaptured by Byzantine exiles in 1261, the empire was now little more than a large city-state. In the 14th century the Ottoman Turks began to encroach; their extended siege of Constantinople ended in 1453, when the last emperor died fighting on the city walls and the area came under Ottoman control.
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After siding with Pescennius Niger against the victorious Septimius Severus, the city was besieged by Roman forces and suffered extensive damage in 196 AD. Byzantium was rebuilt by Septimius Severus, now emperor, and quickly regained its previous prosperity. The location of Byzantium attracted Roman Emperor Constantine I who, in 330 AD, refounded it as Nova Roma. After his death the city was called Constantinople (Greek Κωνσταντινούπολις or Konstantinoupolis) ('city of Constantine'). It remained the capital of the Eastern Roman Empire, which was later called the Byzantine Empire by historians.
This combination of imperialism and location would affect Constantinople's role as the crossing point between two continents: Europe and Asia. It was a commercial, cultural, and diplomatic magnet. With its strategic position, Constantinople could control the route between Asia and Europe, as well as the passage from the Mediterranean Sea to the Black Sea.
On May 29, 1453, the city fell to the Ottoman Turks, and, once again, became the capital of another powerful state, the Ottoman Empire. The Turks called the city Istanbul (though not officially renamed until 1930) and it has remained Turkey's largest (and arguably its most important) city, although Ankara is now the capital.
Byzantium first produced coins with the crescent and star symbol in the 4th century BC. According to legend, this was to honour the moon-goddess Hecate, who the inhabitants believed had saved the city from attack by Philip II of Macedon in 340-339 BC. In 330 AD Constantine I added the Virgin Mary's star to the flag. Byzantium would then also be the first attested nation or empire to use the combination of the crescent moon and star together as an emblem.
The crescent moon and star was not completely abandoned by the Christian world after the fall of Constantinople. To date the official flag of the Orthodox Patriarch of Jerusalem is a labarum of white, a church building with two towers, and on either side of the arms, at the top, are the outline in black of a crescent moon facing center and a star with rays.