Montreux Convention Explained, the 1936 Treaty That Still Decides Which Warships Enter the Black Sea

Every warship that wants to pass the Bosporus is governed by a treaty signed in a Swiss lakeside town in July 1936, before radar, before aircraft carriers mattered, and before NATO existed. It has outlived the League of Nations, the Soviet Union and every government that signed it. In the fifth year of Russia's full-scale war on Ukraine, it is still the most important document in Black Sea security.

The Montreux Convention Regarding the Regime of the Straits was signed on 20 July 1936 by Turkey, Britain, France, the Soviet Union and several other states. It replaced the 1923 Lausanne arrangement, which had demilitarised the straits and put them under an international commission. Montreux handed control back to Turkey and let it fortify the shores again. In return, Ankara promised to keep the straits open on the terms the treaty sets out.

Painting: Amedeo Preziosi, public domain, via Wikimedia Commons

Merchant ships pass, warships ask

For merchant ships the rule is simple. In peacetime any ship, under any flag, carrying any cargo, may pass day or night. Turkey can't charge a toll for passage itself, only fees for lighthouses, sanitary control and rescue services. That clause is why Ankara has spent the past four years raising its "lighthouse fee" rather than charging a transit price (see the traffic and transit rules).

Warships are a different matter, and here the treaty draws a hard line between Black Sea states and everyone else. Russia, Ukraine, Romania, Bulgaria and Georgia may send capital ships of any size through, one at a time with no more than two destroyers as escort, and their submarines may pass on the surface, by day, to return to base or go for repairs. Navies from outside the Black Sea get far less. They must notify Turkey in advance, may send only light surface ships, can't have more than 15,000 tons in transit at once, and can't keep more than 30,000 tons in the Black Sea in total (45,000 in some conditions). Each ship can stay 21 days, no more.

Aircraft carriers are not among the classes the treaty lets through. The Soviet Union got its carriers out of the Black Sea shipyards at Mykolaiv by calling them "aircraft-carrying cruisers". Nobody else has tried.

Article 19 and the closure of February 2022

The clauses that matter now cover war. Under Article 19, if Turkey is neutral, warships of the countries at war may not use the straits, except to return to their home base. Under Articles 20 and 21, if Turkey is at war or feels threatened by imminent war, it decides alone who passes.

On 28 February 2022, four days after Russia invaded Ukraine, Turkey declared that a state of war existed and applied the convention. It warned every country, from the Black Sea or beyond, against sending warships through. Russian ships that were outside the Black Sea could go home and stay home. Turkey refused a Russian request in early 2022 to send four more warships in.

The closure has cut both ways. It helped Ukraine in the first months by stopping Russia from reinforcing its Black Sea Fleet from the Mediterranean and the Baltic. It has also kept Ukraine's own new ships out. Britain gave Ukraine two Sandown-class minehunters, and in January 2024 Turkey refused to let them through. They are still in Portsmouth, as of October 2026, alongside two Alkmaar-class minehunters from Belgium and the Netherlands, and a fifth the Dutch planned to hand over in June 2026. The Black Sea, meanwhile, is full of drifting mines.

Erdoğan's treaty

Turkey has applied the convention by the letter, and on that narrow point Ankara deserves credit. The treaty is also the one piece of Turkish foreign policy that predates Recep Tayyip Erdoğan and has held steady through him, which says more about the treaty than about him.

Erdoğan treats it as personal leverage. In Sochi in September 2023 he told Vladimir Putin that the Black Sea was "our Black Sea", while Turkey became the largest buyer of Russian oil products and Russia-linked tankers kept sailing past Istanbul unhindered. He has kept non-Black Sea NATO navies out, as the treaty requires, and used that to present himself to Moscow as a reliable gatekeeper and to the West as indispensable.

He has also shown what happens to Turks who defend the convention without his permission. In April 2021, more than a hundred retired admirals signed an open letter warning that his Canal Istanbul project could open the Montreux regime to challenge. Erdoğan said the letter had "implications of a coup", and ten of the admirals were detained within a day. A man who jails retired officers over a letter about a 1936 treaty is not guarding it for the nation. He's guarding it for himself.

What comes next

The convention has no expiry date; any party can give two years' notice to leave, and none has. The pressure on it is rising anyway. Since early October 2026, suspected Russian drone strikes on merchant ships off Romania and Bulgaria have killed or left missing dozens of sailors (more in Black Sea war and shipping), and Bucharest and Sofia want a stronger naval presence. Under Montreux, the US Navy can't send a carrier and can't stay longer than three weeks. Turkey's answer, announced in April 2026, is an Istanbul-based maritime command and a Black Sea task force run by itself, Romania and Bulgaria.

Any post-war settlement in the Black Sea will run through this treaty, and through Ankara.