Kocaeli

i.The Gulf of İzmit and the North Anatolian Fault

Kocaeli is built at the head and along the southern shore of the Gulf of İzmit — the long, deep, east-west-running arm of the eastern Marmara that drives fifty kilometres inland from the Bosphorus to its head at modern İzmit. The gulf is the natural eastern continuation of the Marmara basin; its waters are deep, its harbours sheltered, and the surrounding plains and slopes have been the principal industrial-and-residential corridor of the eastern Marmara region since the 19th century. The country to the north of the gulf rises to the Samanlı Dağları (1,600 m); the southern shore opens onto the Yalova-Bursa plain.

The defining geological feature of the province is the North Anatolian Fault Zone (Kuzey Anadolu Fay Hattı), which runs immediately south of the gulf in a roughly east-west line through the central districts of the province. This is the great right-lateral strike-slip fault that runs from Karlıova in the east to the Saros Gulf in the west, and which has produced the catastrophic Marmara earthquakes of 1509, 1719, 1754, 1894, and most recently of 17 August 1999.

ii.Astacus and Nicomedia — the Foundations

The first historic-period city on the gulf was Astacus (or Olbia), founded in the 8th century BCE as a colony of the Doric Greek city of Megara. Astacus was a working but not large Greek city on the gulf, and was destroyed in the early 3rd century BCE during the wars between the Hellenistic successor kingdoms. The successor city was founded on a new site at the head of the gulf in 264 BCE by King Nicomedes I of Bithynia — one of the smaller Hellenistic kingdoms of northwestern Anatolia, ruled by a Thraco-Phrygian dynasty that had separated from the Seleucid empire two generations earlier — and named for him: Nikomédeia, Nicomedia.

Nicomedia became the capital of the Bithynian kingdom, and through the 3rd and 2nd centuries BCE one of the major cities of northwestern Anatolia. The harbour was the principal Bithynian naval base; the city's mint produced one of the largest Hellenistic-period coinages in the region; the surrounding country was the heart of the Bithynian agricultural economy.

iii.Hannibal at Libyssa (183 BCE)

One famous classical episode of the Bithynian kingdom concerns the Carthaginian general Hannibal Barca, who in the years after his defeat by Rome at Zama (202 BCE) and the imposition of the Treaty of Apamea on the Seleucid empire (188 BCE), found himself, in 195 BCE, an exile at the Bithynian court. King Prusias I of Bithynia gave Hannibal refuge, and the Carthaginian general served the Bithynian state as a military advisor for several years. In 183 BCE, with Roman emissaries pressing King Prusias for his surrender, Hannibal — by then in his mid-sixties — took poison at the small Bithynian town of Libyssa, on the northern shore of the gulf at the eastern edge of modern Gebze. He was buried there. A monument near Gebze's modern Hannibal Park marks the traditional site; the original tomb has not been definitively located.

iv.Diocletian's Eastern Capital (284 CE)

The transformation of Nicomedia from a regional Hellenistic-Roman city into a great metropolis came in 284 CE, when the new emperor Diocletian — recently elevated by the eastern legions — chose Nicomedia as his principal eastern residence. Under Diocletian and his Tetrarchic colleagues the city became the eastern capital of the late Roman empire and, in the contemporary historian Lactantius's account, "the fourth city of the world" after Rome, Antioch, and Alexandria. Diocletian built a vast imperial palace on the slope above the gulf, a hippodrome, new walls, a Senate building, a mint, and a series of monumental fora. The palace was the place from which Diocletian announced his abdication on 1 May 305 — the only Roman emperor to retire voluntarily from the throne.

Under Constantine I, Nicomedia continued briefly as the principal eastern residence; Constantine was baptised here on his deathbed in 337 CE by his cousin Eusebius of Nicomedia, just before the new eastern capital at Constantinople superseded it (see our İstanbul essay). The First Council of Nicaea (325 CE) was held forty kilometres south of Nicomedia, at Nicaea (modern İznik in Bursa province), which under the Tetrarchy was effectively the suburban summer capital of the imperial court.

v.The Great Earthquake of 358 and the Decline

The first great seismic catastrophe of the city's history came on 24 August 358 CE, when a North-Anatolian-Fault earthquake — described in detail by the historian Ammianus Marcellinus — destroyed much of Diocletian's monumental capital. The walls, the palaces, the hippodrome, and a large part of the population were lost in a single morning. Constantinople, now firmly established as the eastern capital, eclipsed the damaged Nicomedia decisively. Subsequent earthquakes — particularly that of 554 — completed the demolition of the late-antique city. By the early Byzantine centuries Nicomedia had reverted to a fortified provincial centre.

vi.Byzantine Continuation and the Seljuk-Crusader Period

Through the long Byzantine centuries Nicomedia remained a working provincial capital — the centre of the Optimaton and later the Opsikion themes, with a citadel on the slope above the gulf and a small but persistent harbour economy. After the Battle of Manzikert in 1071, Seljuk raiding bands reached the gulf, and in the early 1080s the Seljuk Sultan of Rûm Süleyman Shah briefly took the city. The First Crusade returned it to Byzantine control in 1097; through the 12th and 13th centuries it was, alternately, a frontier post of the contracting Byzantine empire and a target of Turkish raids from the interior. After the Latin sack of Constantinople in 1204 it formed part of the Empire of Nicaea under the Laskarid dynasty before returning to Byzantine control in 1261.

vii.The Ottoman Conquest of 1337

The Ottoman capture of Nicomedia came as the final consolidation of the early Ottoman state's hold on the southern shore of the Marmara. After the conquest of Bursa (1326) and Nicaea (1331), Orhan Gazi dispatched his commander Akçakoca Bey — the eponym of the modern Black Sea province of Düzce-Akçakoca — to take Nicomedia. The city fell to the Ottomans in 1337 after a long siege. The new name İznikmid shifted, through the next two centuries, to the modern İzmit, the form the city has carried since. The province as a whole took its name from Kocaeli ("the realm of Kocaman") — said in local tradition to refer to a follower of Orhan Gazi, Kocaman Bey, who commanded the early-14th-century raids on the gulf. The Ottoman period gave the city its centre at the harbour and the surviving older monuments — the Yenicuma Camii (1334, the first Ottoman mosque in the city), the Pertev Paşa Külliyesi (1579, by Sinan in his late years), and the late-Ottoman government house.

viii.Late Ottoman İzmit and the Imperial Naval Yard

İzmit's importance through the long Ottoman centuries was steady but not dramatic — a working provincial sancak centre, the principal eastern Marmara port, and an industrial-and-textile centre of the late 19th century after the opening of the Haydarpaşa-İzmit railway in 1873. The decisive 19th-century investment came at Gölcük on the southern shore of the gulf, where the Ottoman state built the imperial Gölcük Tersanesi (the Gölcük Naval Yard) in the 1880s; the yard became, under the Republic, the principal naval base and shipbuilding facility of the Turkish Navy, and remains so today. The 1924 establishment of the modern Republican administrative structure made Kocaeli a province; through the 20th century the city grew steadily, with the major industrial investments of the 1960s and 70s — the Petkim petrochemical complex at Yarımca, the Tüpraş oil refinery at Körfez, the Ford Otosan automobile plant at Gölcük — transforming it into one of the country's leading industrial provinces.

ix.The Modern Industrial Province

Under the TÜİK 2024 address-based registration count the province population was 2,130,006 — the sixth largest in Türkiye. The metropolitan municipality covers twelve districts. The largest by population is Gebze (~412,000), the easternmost district and the principal industrial centre on the Marmara-İstanbul corridor; followed by İzmit (~381,000, the administrative capital and historic centre); Darıca (~231,000); Gölcük (~179,000, with the Naval Yard and the Ford automotive plant); and Çayırova (~158,000). Kocaeli is the leading manufacturing province of Türkiye by output, with petrochemicals, automotive, white goods, and machine industries together accounting for over fifteen percent of national industrial production.

The province is the seat of Kocaeli Üniversitesi (founded 1992) and Gebze Teknik Üniversitesi (founded 1992 as the Gebze Institute of Technology). The deep-water terminals at Derince, Körfez, and Yarımca handle a substantial share of national container and bulk traffic. The principal post-1999 urban-redevelopment project — the rebuilding of the Seka Park waterfront in central İzmit, opened in 2009 — has given the city a long, distinguished pedestrian quayside.

x.What to See, in Order

The walking circuit of historic İzmit runs along the harbour. From the central İstiklal Caddesi the route runs to the Pertev Paşa Külliyesi (Sinan, 1579), the Saatçi Ali Efendi Camii (an Ottoman-period mosque in the harbour district), the Süleyman Paşa Hamamı (14th century, the oldest standing Ottoman bath in the city), and the small Kocaeli Arkeoloji Müzesi on the waterfront, with its collection of Bithynian and Roman-period inscriptions and statuary recovered from the lower slopes of the Diocletian palace complex. The visible remains of the Roman city are modest — the foundations of the imperial palace, some sections of the city wall — but identifiable on a walking tour.

For the wider province, the principal excursions reach Gebze for the traditional site of Hannibal's burial at Libyssa (with the small commemorative Hannibal Anıtı in the modern Gebze park), the Çoban Mustafa Paşa Külliyesi (1523, by the Ottoman vizier whose name the complex carries), and the Eskihisar Castle on the rocky promontory overlooking the gulf; Gölcük for the Naval Yard's small Tarih Müzesi (with the original buildings of the 1880s imperial yard); and the small Roman-period port of Hereke on the northern shore (now famous for the Hereke carpets first commissioned by Abdülmecid I in 1843).

The Marmara gulf of Bithynian Nicomedia, Diocletian's eastern capital, Hannibal's grave, and the great industrial province rebuilt after 17 August 1999.

For the Marmara provinces immediately to the west, see İstanbul and the planned Yalova essay; for the south side of the Marmara, see Bursa; for the eastern Marmara province paired with Kocaeli, see Sakarya. For Türkiye's Marmara basin and its faults in the wider sense, visit our sister site CountryOfTurkey.com.