Bosphorus

The Bosphorus is a 31-kilometre natural strait in northwestern Turkey connecting the Black Sea to the Sea of Marmara, and via the Dardanelles, to the Mediterranean. Istanbul straddles both banks, with a metropolitan population of around 17 million. The strait narrows to 700 metres between Kandilli Point and Asiyan, with an average depth of 65 metres. Approximately 40,000 to 48,000 vessels transit annually, making it one of the world's busiest navigable chokepoints and a critical route for Black Sea energy and grain exports. Total Turkish Strait traffic hit a record 84,640 ships across the Bosphorus and Dardanelles combined in 2025, continuing to climb despite warship restrictions.

Turkey controls passage under the 1936 Montreux Convention, which guarantees freedom of transit for commercial vessels in peacetime. For warships, the convention imposes strict limits: no single group may exceed 15,000 metric tons; no more than nine warships may transit simultaneously; only Black Sea states may transit submarines; and under Article 19, Turkey may deny passage to warships of nations at war if Turkey itself is not a belligerent. On 27-28 February 2022, three days after Russia's Ukraine invasion, Ankara formally invoked Article 19 and closed the strait to warships of all belligerents. Russia's existing Black Sea Fleet was not trapped (vessels already in the Black Sea could return home), but NATO reinforcement was blocked simultaneously. Turkey classified the Ukraine conflict as a "war" to activate the provision; in January 2024 it also blocked British minehunters designated for Ukraine from transiting.

The strait's significance extends beyond the Russia-Ukraine conflict. It is the sole maritime route for Black Sea littoral states to reach global markets. Russia's Black Sea energy and grain exports from Novorossiysk continue via commercial vessels; the Montreux restrictions apply only to warships. Ukraine's drone campaign has extended into the Black Sea, damaging Russian naval assets and compressing the fleet's operating area; one documented incident saw a sanctioned Russian LNG carrier destroyed off the Libyan coast in March 2026, attributed to a Ukrainian drone that had extended its range beyond the strait. Any disruption to commercial Bosphorus traffic would affect grain, energy, and container shipping across the Mediterranean basin.