The Turkish Straits System, Why the Bosporus Only Works Together With the Marmara and the Dardanelles

A tanker leaving Novorossiysk for the Mediterranean doesn't pass one strait. It passes a system about 370 kilometres long: the Bosporus through Istanbul, the Sea of Marmara, and then the Dardanelles, the 61-kilometre channel that Turks call the Çanakkale Strait. International law treats the three as one waterway, the Turkish Straits, and so does every navy that has tried to force or hold them.

The Bosporus gets the attention because it runs through a city of 16 million. The Dardanelles is busier. In 2025 Turkey counted 44,468 transits there against 40,172 in the Bosporus, because ships bound for the Marmara's own ports, Ambarlı, the İzmit gulf, Gemlik and Tekirdağ, use the Dardanelles without ever reaching Istanbul. The same pattern held in the first half of 2026.

Photo: Zafer, public domain, via Wikimedia Commons

Three waterways, one problem

The two straits are cousins. Both are drowned river valleys, both carry the same two-layer flow (fresher Black Sea water on top heading south, saltier Mediterranean water underneath heading north), and both have a narrow point where whoever holds the shores can close them. In the Bosporus it's the stretch below Rumeli Hisarı, about 700 metres wide. In the Dardanelles it's the Narrows between Çanakkale and Kilitbahir, about 1.2 kilometres.

The Marmara in between, about 280 kilometres long, is Turkey's inland sea. It gives ships room to anchor and wait, and it gives Turkey defence in depth. An enemy fleet has to get through both narrows, and the Marmara between them, to reach Istanbul.

Five centuries of fighting over the gates

Mehmed II understood the system. In 1452 he built Rumeli Hisarı on the Bosporus to cut Constantinople off from the Black Sea, and a decade after taking the city he locked the other gate with the fortresses of Kilitbahir and Kale-i Sultaniye on either side of the Dardanelles narrows. For the next four centuries the Ottoman Empire could close the Black Sea at will, which made the straits Russia's permanent grievance. The 1774 Treaty of Küçük Kaynarca gave Russian merchant ships the right to pass. The 1841 London Straits Convention closed the straits to all foreign warships in peacetime, an arrangement Britain liked because it kept the Russian fleet bottled up.

In 1915 Britain and France tried to take the straits by force. On 18 March their battleships went into the Dardanelles and ran into Ottoman mines and guns; three were sunk and three more crippled. The landings at Gallipoli that followed cost both sides hundreds of thousands of casualties and ended in Allied withdrawal in January 1916. Mustafa Kemal, later Atatürk, made his name there.

After the war the victors demilitarised the shores and put an international commission in charge under the 1923 Lausanne Treaty. Turkey won them back in 1936 at Montreux (see the Montreux Convention explained). Stalin tried again in 1945 and 1946, demanding Soviet bases and joint control. Turkey refused, the US sent warships to the Mediterranean, and in 1952 Turkey joined NATO. The straits were a large part of the reason.

The bridge over the Dardanelles

On 18 March 2022, the 107th anniversary of the naval battle, Recep Tayyip Erdoğan opened the 1915 Çanakkale Bridge across the Dardanelles near Gelibolu. Its main span of 2,023 metres, a number chosen for the republic's centenary, made it the longest suspension span in the world. It cost about 2.5 billion euros.

It is also a fair picture of how Erdoğan builds. The bridge was financed as a build-operate-transfer deal in which the state guaranteed the operators 45,000 vehicles a day; on the first weekend with tolls about 6,000 crossed. The shortfall is paid from the Turkish budget, by taxpayers who mostly never use the bridge. The same model covers the Yavuz Sultan Selim Bridge over the Bosporus and Istanbul's new airport.

Why it still matters

Today the system is under strain at both ends. In the north, the Black Sea has become a war zone where merchant ships are sunk by drones (Black Sea war and shipping). Through the middle, Russia's shadow tankers move oil past Istanbul and Çanakkale every day (the shadow fleet). And in Ankara, a government that has gutted the country's institutions holds the keys to one of the world's oldest chokepoints.

Whoever controls the Turkish Straits controls the Black Sea's only door to the world. That hasn't changed since 1452.