Istanbul is the largest city in Turkey, constituting the country's economic, cultural, and historical heart. With a population of , the city forms one of the largest urban agglomerations in Europe and is among the largest cities in the world by population within city limits. Istanbul's vast area of is coterminous with Istanbul Province, of which the city is the administrative capital. Istanbul is a transcontinental city, straddling the Bosphorus—one of the world's busiest waterways—in northwestern Turkey, between the Sea of Marmara and the Black Sea. Its commercial and historical center lies in Europe, while a third of its population lives in Asia.
Founded on the Sarayburnu promontory around 660 BC as Byzantium, the city now known as Istanbul developed to become one of the most significant cities in history. For nearly sixteen centuries following its reestablishment as Constantinople in 330 AD, it served as the capital of four empires: the Roman Empire (330–395), the Byzantine Empire (395–1204 and 1261–1453), the Latin Empire (1204–1261), and the Ottoman Empire (1453–1922). It was instrumental in the advancement of Christianity during Roman and Byzantine times, before the Ottomans conquered the city in 1453 and transformed it into an Islamic stronghold and the seat of the last caliphate. Although the Republic of Turkey established its capital in Ankara, palaces and imperial mosques still line Istanbul's hills as visible reminders of the city's previous central role.
Istanbul's strategic position along the historic Silk Road, rail networks to Europe and the Middle East, and the only sea route between the Black Sea and the Mediterranean have helped foster an eclectic populace, although less so since the establishment of the Republic in 1923. Overlooked for the new capital during the interwar period, the city has since regained much of its prominence. The population of the city has increased tenfold since the 1950s, as migrants from across Anatolia have flocked to the metropolis and city limits have expanded to accommodate them. Arts festivals were established at the end of the 20th century, while infrastructure improvements have produced a complex transportation network.
Seven million foreign visitors arrived in Istanbul in 2010, when it was named a European Capital of Culture, making the city the world's tenth-most-popular tourist destination. The city's biggest draw remains its historic center, partially listed as a UNESCO World Heritage Site, but its cultural and entertainment hub can be found across the city's natural harbor, the Golden Horn, in the Beyoğlu district. Considered a global city, Istanbul hosts the headquarters of many Turkish companies and media outlets and accounts for more than a quarter of the country's gross domestic product. Hoping to capitalize on its revitalization and rapid expansion, Istanbul is currently bidding for the 2020 Summer Olympics.
Neolithic artifacts, dating back to the 7th millennium BC and uncovered by archaeologists at the beginning of the 21st century AD, indicate Istanbul's historic peninsula was settled earlier than previously thought and before the Bosphorus was even formed. Before the discovery, conventional wisdom held that Thracian tribes, including the Phrygians, began settling on the Sarayburnu in the late 6th millennium BC. On the Asian side, artifacts originating around the 4th millennium BC have been found in Fikirtepe (within Kadıköy). The same location was the site of a Phoenician trading post at the beginning of the 1st millennium BC as well as the town of Chalcedon, which was established in c. 680 BC.
However, the history of Istanbul generally begins around 660 BC, when settlers from Megara, under the command of King Byzas, established Byzantium on the European side of the Bosphorus. The settlers proceeded to build an acropolis adjacent to the Golden Horn on the site of the early Thracian settlements, fueling the nascent city's economy. The city experienced a brief period of Persian rule at the turn of the 5th century BC, but the Greeks recaptured it during the Greco-Persian Wars. Byzantium then continued as part of the Athenian League and its successor, the Second Athenian Empire, before ultimately gaining independence in 355 BC. Long allied with the Romans, Byzantium officially became a part of the Roman Empire in 73 AD.
Byzantium's decision to side with the usurper Pescennius Niger against Roman Emperor Septimius Severus cost it dearly; by the time it surrendered at the end of 195 AD, two years of siege had left the city devastated. Still, five years later, Severus began to rebuild Byzantium, and the city regained—and, by some accounts, surpassed—its previous prosperity.
Rise and fall of Constantinople
Constantine the Great effectively became the emperor of the whole of the Roman Empire in September 324. Two months later, Constantine laid out the plans for a new, Christian city to replace Byzantium. As the eastern capital of the empire, the city was named Nea Roma; however, most simply called it Constantinople, a name that persisted into the 20th century. Six years later, on 11 May 330, Constantinople was proclaimed the capital of an empire that eventually became known as the Byzantine Empire or Eastern Roman Empire.
Constantinople began to decline after the Fourth Crusade, during which it was sacked and pillaged. The city subsequently became the center of the Latin Empire, created by Catholic crusaders to replace the Orthodox Byzantine Empire. However, the Latin Empire was short-lived, and the Byzantine Empire was restored, albeit weakened, in 1261. Constantinople's churches, defenses, and basic services were in disrepair, and its population had dwindled to a hundred thousand from up to half a million during the 8th century.
Various economic and military policies instituted by Andronikos II, such as the reduction of military forces, weakened the empire and left it more vulnerable to attack. In the mid-14th century, the Ottoman Turks began a strategy of taking smaller towns and cities over time, cutting off Constantinople's supply routes and strangling it slowly. Finally, on 29 May 1453, after an eight-week siege (during which the last Roman emperor, Constantine XI, was killed), Sultan Mehmed II "the Conqueror" captured Constantinople and declared it the new capital of the Ottoman Empire. Hours later, the sultan rode to the Hagia Sophia and summoned an imam to proclaim the Islamic creed, converting the grand cathedral into an imperial mosque.
Ottoman and Turkish era
Following the fall of Constantinople, Mehmed II immediately set out to revitalize the city, by then also known as Istanbul. He urged the return of those who had fled the city during the siege, and forcibly resettled Muslims, Jews, and Christians from other parts of Anatolia. The sultan invited people from all over Europe to his capital, creating a cosmopolitan society that persisted through much of the Ottoman period. Meanwhile, Mehmed II repaired the city's damaged infrastructure, began to build the Grand Bazaar, and constructed Topkapı Palace, the sultan's official residence.
The Ottomans quickly transformed the city from a bastion of Christianity to a symbol of Islamic culture. Religious foundations were established to fund the construction of grand imperial mosques, often adjoined by schools, hospitals, and public baths. The Ottoman Dynasty claimed the status of caliphate in 1517, with Istanbul remaining the capital of this last caliphate for four centuries. Suleiman the Magnificent's reign from 1520 to 1566 was a period of especially great artistic and architectural achievement; chief architect Mimar Sinan designed several iconic buildings in the city, while Ottoman arts of ceramics, calligraphy, and miniature flourished. The total population of Istanbul amounted to 570,000 by the end of the 18th century.
A period of rebellion at the start of the 19th century led to the rise of the progressive Sultan Mahmud II and eventually to the Tanzimat period, which produced political reforms and allowed new technology to be introduced to the city. Bridges across the Golden Horn were constructed during this period, and Istanbul was connected to the rest of the European railway network in the 1880s. Modern facilities, such as a stable water network, electricity, telephones, and trams, were gradually introduced to Istanbul over the following decades, although later than to other European cities. Still, the modernization efforts were not enough to forestall the decline of the Ottoman Empire.
In the early 20th century, the Young Turk Revolution disposed of Sultan Abdul Hamid II and a series of wars plagued the ailing empire's capital. The last of these, World War I, resulted in the British, French, and Italian occupation of Istanbul. The final Ottoman sultan, Mehmed VI, was exiled in November 1922; the following year, the occupation of Istanbul ended with the signing of the Treaty of Lausanne and the recognition of the Republic of Turkey, declared by Mustafa Kemal Atatürk.
In the early years of the republic, Istanbul was overlooked in favor of Ankara, selected as Turkey's capital to distance the new, secular country from its Ottoman history. However, starting from the late 1940s and early 1950s, Istanbul underwent great structural change, as new public squares, boulevards, and avenues were constructed throughout the city, sometimes at the expense of historical buildings. The population of Istanbul began to rapidly increase in the 1970s, as people from Anatolia migrated to the city to find employment in the many new factories that were built on the outskirts of the sprawling metropolis. This sudden, sharp rise in the city's population caused a large demand for housing development, and many previously outlying villages and forests became engulfed into the metropolitan area of Istanbul.