Emperor Diocletian who ruled the Roman Empire from 284 to 305 CE believed that the empire had become too big for one person to rule. Therefore, he divided the empire into two parts ( east and west) and introduced the system of tetrarchy ( rule of four) with an emperor (augustus) and co-emperor (caesar)  in both the parts. Constantine I or Constantine the Great reunited Roman empire after winning a prolonged civil war. His reign is known for two major events – moving the imperial capital to the small ancient city of Byzantium and Christianization of the Roman empire. Byzantium was renamed “Nova Roma” ( New Rome). In 330 CE the capital city was dedicated to Constantine and renamed “Constantinople”. The empire once again got divided into two after the death of emperor Theodosius I in 395 CE. The western part collapsed in 476 CE due to onslaught of Germanic tribes but the eastern part known as Byzantine empire continued to flourish. Constantinople remained the capital of Byzantine empire.

Sea of Marmara and the Golden Horn Bay surrounded two-thirds of the city of Constantinople and emperor Theodosius II secured it from the land side by building defensive walls which later became famous as Theodosian Walls. This city was located on a peninsula between the Black Sea and the Aegean Sea and it connected two continents – Asia and Europe. Constantinople became melting pot of races and cultures due to its unique geographic location straddling Europe and Asia. It continued to be governed by Roman law but Greek became its primary language and Christianity its state religion.  The city acquired fame for its architectural masterpieces, such as Hagia Sophia ( it was famous for its massive dome and it was considered to be the world’s largest building until 1520 when church of Seville was built), Golden Gate of the Theodosian Walls, the Hippodrome and opulent aristocratic palaces.  Byzantine empire perhaps reached its peak during the reign of Justinian I but crisis of leadership emerged after his death and it ended up being run by several weak and unpopular rulers. The empire lost significant territory throughout this chaotic period.

The rise of Islamic Caliphate and its victory against Byzantine army at the battle of Yarmouk in 636 CE completely shifted the balance of power in the region. Syria was lost to Arabs and by 646 CE Arabs had snatched Mesopotamia, Palestine and Egypt from Byzantines.  However, the Byzantines managed to hold on to its capital city of Constantinople. The locational advantage and massive protective Theodosian Walls of Constantinople enabled it to withstand time and again concerted attacks from both sea and land. The Byzantine empire continued to survive in a minor form.   The fortune of Byzantine empire revived under the rule of Macedonian dynasty founded by Basil I in the year 867CE.  Macedonian period is considered as the golden age of the Byzantine empire. A long period of military struggle for survival ended as Byzantine empire went on the offensive against its enemies. Art and literature once again flourished under the new found security and stability. Paintings from this period would have a strong influence on the artists of the Italian Renaissance. Political boundary of the empire expanded and Basil II, great-great grandson of the founder Basil I ruled an empire that was the undisputed superpower of the day.  

        In the year 1071 CE, Seljuk Turks defeated Byzantine army at the battle of Manzikert which marked the beginning of the irreversible process of decline of the Byzantine empire. Byzantine empire needed support of west European Christian powers to survive the onslaught of Turkish invaders. When Seljuk Turks took control of Jerusalem in 1087 CE, the Byzantine emperor used it as an opportunity to gain support of Christian powers which led to the First Crusade. The First Crusade was successful in recapturing Jerusalem in the year 1099 but it had many negative consequences from the Byzantine point of view. Unruly group of crusaders caused havoc in Byzantine territory, there were outbreaks of fighting between crusaders and Byzantine forces causing rupture in its relation with western Christian powers. Constantinople got sacked in the year 1204 CE by the crusaders during the fourth crusade.  In 1261 CE, forces from Empire of Nicaea, the centre of the Byzantines-in-exile retook Constantinople and re-established Byzantine empire. The new ruler Michael VIII Palaeologos devoted much of his efforts to rebuilding Constantinople and was hailed as the New Constantine. However, the city remained a shadow of its former self as the expense of reconstruction devalued its currency and the Byzantine aristocracy failed to compete with Venetians and Genoese. Byzantine empire was now little more than a large city state. Mongol invasion in 1243 led to the decline of Seljuk Sultanate of Rum, the chief rival of Byzantine empire in Asia Minor.  Seljuk Sultanate was broken into several independent mini-states known as beyliks. Osman I, ruler of one of these beyliks, founded Ottoman empire which very soon became the most potent threat to the Byzantine empire. Ottoman victory in the Battle of Kosovo in 1389 resulted in complete encirclement of the Byzantine empire by Turkish armies. Ottomans now turned their attention towards capture of Constantinople. After many unsuccessful attempts, the last and final siege of Constantinople began on April 2, 1453. Constantinople received little help from west European powers due to the schism between Orthodox Church of Byzantine and the Roman Catholic Church. Sultan Mehmed II hired Orban, a Hungarian engineer, to build a massive cannon which was able to breach the famous Theodosian Wall. The Byzantines were hopelessly outnumbered in men, ships and weapon. Constantinople finally fell to the Ottomans on May 29, 1453. Fall of Constantinople marked the end of the Roman empire, the last surviving link between the ancient and the medieval world. There was a flood of refugees from Constantinople to various Italian city-states. These refugees included poets, musicians, architects, philosophers and scientists who acted as catalysts of Renaissance in Italy which very soon spread across Europe. Fall of Byzantine empire was a big blow to the Christian world but it also led to Renaissance which made Western Europe the dominant world power. Constantinople, now known as Istanbul, is the largest city in Turkey serving as the country’s economic, cultural and historic hub.