Geopolitics › Global issues & flashpoints › Maritime chokepoints
Bosporus and the Turkish Straits
Montreux Convention 1936· Resource or chokepoint
FactTreaties, membership, measured data, events.
The Bosporus and Dardanelles, linking the Black Sea to the Mediterranean through Istanbul, governed by the 1936 Montreux Convention.
See it on the Geopolitics map →The picture
The Bosporus is about 31 km long and as narrow as about 700 m, running through a city of some 16 million. With the Sea of Marmara and the Dardanelles, it is the Black Sea's only outlet: Russian and Kazakh oil and Ukrainian and Russian grain pass through. The Montreux Convention guarantees merchant ships free passage in peacetime, limits warships from outside the Black Sea, and lets Turkey bar warships of countries at war.
Why it matters
Control of this gate gives Turkey leverage over Russia and NATO alike; its 2022 closure to warships boxed in Russia's Black Sea Fleet during the Ukraine war.
Key facts and numbers
The Montreux Convention text, Turkish government notices on its 2022 invocation, and Turkish Straits vessel-traffic statistics.
What to watch
Whether Erdoğan's stalled Kanal Istanbul, a planned bypass canal, would fall outside Montreux rules, as critics fear, and whether it is ever built.
Connections (3)
▸Russia's oil 'shadow fleet' carries oil through Bosporus and the Turkish StraitsFact
The relationship
Tankers loading at Russia's Black Sea port of Novorossiysk must pass the Bosporus and Dardanelles to reach world markets. Under the 1936 Montreux Convention, Turkey must let merchant ships through in peacetime, though since December 2022 it has demanded proof of insurance from tankers.
Since when and why
Istanbul's Bosporus, ongoing; in November 2025 Ukrainian sea drones hit two sanctioned tankers, Kairos and Virat, in the Black Sea off Turkey's coast, alarming Ankara.
How we know
Turkish maritime rules, ship-tracking data on Bosporus transits, and Turkish and Ukrainian statements after the 2025 tanker strikes.
What to watch
Whether Turkey would ever block uninsured shadow-fleet tankers, risking Russian anger and its Montreux obligations, is an open question.
▸Turkey (Türkiye) controls Bosporus and the Turkish StraitsFact
The relationship
Istanbul straddles the Bosporus, making Turkey the gatekeeper of all Black Sea sea trade. The 1936 Montreux Convention restored its full control over the straits and lets it regulate warship passage. Ankara used that power in February 2022 to close the straits to warships of countries at war, then helped broker the Black Sea grain deal.
Since when and why
Signed at Montreux, Switzerland, on July 20, 1936; invoked by Turkey on February 28, 2022.
How we know
The convention's text, Turkish Foreign Ministry statements and naval tracking of transits since 2022.
What to watch
Whether Turkey keeps strict neutrality if Russia and NATO ever clash directly, and how a future Kanal Istanbul would square with Montreux.
▸Bosporus and the Turkish Straits were closed to warships during Russia–Ukraine warFact
The relationship
Under the 1936 Montreux Convention, Turkey can bar warships of countries at war. It invoked that rule days after the invasion, so Russia could not reinforce its Black Sea Fleet from other fleets, while Ukrainian missiles and sea drones sank or damaged many ships, forcing the fleet from Crimea to Novorossiysk. Commercial shipping continued.
Since when and why
Ankara, February 28, 2022; Turkey also brokered the 2022–23 Black Sea Grain Initiative with the UN.
How we know
Turkish government statements and the Montreux treaty text; naval tracking shows the ban has held.
What to watch
Whether Turkey keeps the ban after a ceasefire, and how mines and Ukrainian strikes on Russian tankers will affect Black Sea shipping.