Bosphorus

The Bosphorus is a 31-kilometre strait in northwestern Turkey connecting the Black Sea to the SEA of Marmara and, via the Dardanelles, the Mediterranean. It narrows to 700 metres between Kandilli Point and Asiyan, with an average depth of 65 metres, and Istanbul straddles both banks with a metropolitan population of around 17 million. Roughly 40,000 to 48,000 vessels transit annually; combined traffic across the Bosphorus and Dardanelles hit a record 84,640 ships in 2025.

Turkey controls passage under the 1936 Montreux Convention, which guarantees peacetime transit for commercial vessels but caps warship tonnage, numbers and, under Article 19, lets Ankara bar belligerent warships if Turkey itself stays out of the war. Ankara invoked that clause on 27-28 February 2022, three days after Russia's invasion of Ukraine, closing the strait to warships from both sides; Russia's existing Black Sea Fleet was not trapped, but NATO reinforcement was blocked at the same stroke. In January 2024 Turkey applied the same closure to British minehunters bound for Ukraine.

the strait remains the only maritime route for Black Sea littoral states to reach world markets, so Russian energy and grain exports from Novorossiysk still move through it as commercial cargo, unaffected by the warship restrictions. Ukraine's drone campaign has since extended into the wider Black Sea and Mediterranean; a sanctioned Russian LNG carrier was destroyed off Libya in March 2026, evidence that risk to shipping now reaches well beyond the strait itself.