Montreux Convention
The 1936 treaty that gave Turkey sovereign control of the Bosphorus and Dardanelles, including the power to close the straits to warships in wartime.
The Montreux Convention Regarding the Regime of the Straits, signed on 20 July 1936 in the Swiss town of Montreux, is the treaty that governs who may sail through the Turkish Straits, the Bosphorus and the Dardanelles, which connect the Mediterranean to the Black Sea. Its core bargain: full Turkish sovereignty over the straits, in exchange for guaranteed passage rules that outside navies have chafed at ever since. Ninety years on, it is still in force, and still decisive.
From demilitarized zone to Turkish waterway
Montreux replaced the straits regime of the 1923 Treaty of Lausanne, which had demilitarized the waterway and placed it under an International Straits Commission of the League of Nations. By the mid-1930s, with Italy expanding and the League decaying, Turkey wanted its chokepoint back. At Montreux it got it. The commission was abolished, Turkey was allowed to remilitarize and fortify the straits, and sovereignty reverted fully to Ankara. The signatories included Turkey, the Soviet Union, Britain, France, Greece, Romania, Bulgaria, Yugoslavia, and Japan.
The rules
The convention treats merchant ships and warships very differently. In peacetime, commercial vessels of any flag pass freely, day or night. Warships face a tiered system that deliberately favors the Black Sea states over outsiders. Navies of non-Black-Sea powers hit hard caps: no more than nine warships at a time, no more than 15,000 tons in transit together, no single ship over 10,000 tons, no aircraft carriers, and a strict 21-day limit on how long they may stay in the Black Sea, where their combined tonnage is capped at 45,000 tons. Black Sea powers get far more latitude and shorter notice periods.
The clauses that matter most are about war. If Turkey is neutral, warships of belligerent states may not pass. If Turkey is itself at war, or believes itself threatened by imminent war, it may close the straits to warships entirely, at its own discretion.
Why it still bites
Montreux froze a status quo that has irritated Russia for generations. It limits how much naval force outside powers can push into the Black Sea, which cuts against NATO, but it caps everyone, and Moscow long resented the constraints on its own fleet's exit. When the Soviet Union pressed Turkey to revise the treaty in the 1940s, Turkish alignment with the West and the Truman Doctrine followed.
The convention returned to headlines in 2022. Days after Russia invaded Ukraine, Turkey formally recognized the conflict as a war and invoked Montreux to close the straits to belligerent warships, barring additional Russian and Ukrainian vessels from entering the Black Sea, though ships returning to their home base were exempt. Meanwhile President Erdogan has pushed Kanal Istanbul, an artificial canal parallel to the Bosphorus. Because a canal is not a "strait," it might fall outside Montreux entirely, letting Turkey charge tolls and set its own terms, and, critics fear, quietly unraveling the treaty that has kept the Black Sea's door half-closed for nearly a century.