Montreux Convention at 90: how a 1936 treaty still blocks NATO from the Black Sea

Four years into Russia's full-scale invasion of Ukraine, the Black Sea has become one of Europe's most contested security frontiers. Yet it remains, in the words of Thomas de Waal of Carnegie Europe, "quite empty of warships." The paradox has a name: the Montreux Convention, signed 90 years ago this month in a Swiss lakeside town. That treaty still decides which vessels may enter the Black Sea, and it is the reason NATO does not maintain a permanent naval presence there.

A single bottleneck and a 1936 compromise

Six countries border the Black Sea: Turkey, Bulgaria, Romania, Ukraine, Russia and Georgia. Three are NATO members; three are not. Two of the latter are at war. Access runs through the Dardanelles and the Bosporus, a waterway that at its narrowest is just 700 metres wide, roughly one-fiftieth the width of the Strait of Hormuz. Whoever controls the Turkish Straits controls the Black Sea's trade and its military balance.

On 20 July 1936, delegates from just under a dozen states gathered at Montreux to end the international oversight imposed after the Ottoman Empire's defeat in the First World War. The resulting convention restored Turkish sovereignty over the straits while guaranteeing free passage for civilian traffic in peacetime. For warships, it created a tiered system: Black Sea littoral states enjoy relatively free transit; non-littoral powers face strict limits on tonnage, fleet size and a 21-day maximum stay. In wartime, if Turkey is not a belligerent, warships of the warring parties are barred entirely, the clause that has shaped the naval war since February 2022.

Turkey's leverage: sovereignty, security and status

"The most privileged Black Sea state is Türkiye itself," said Daria Isachenko of the German Institute for International and Security Affairs. For Ankara, Montreux is not merely a maritime treaty; it is one of the founding documents of the Turkish Republic, embodying sovereignty, security and international status. That makes it politically untouchable. "No one wants to touch it," de Waal noted, "out of concern that it opens a kind of Pandora's box when all the issues have to be renegotiated with Russia." The convention even still references the League of Nations, dissolved 80 years ago.

This gives Turkey a veto over NATO's naval posture. "For NATO, Montreux is a constraint," Isachenko said. "For Türkiye, it is an instrument to keep the balance in the Black Sea." De Waal put it more bluntly: "In the Black Sea, Turkey is very much Turkey first and NATO second." Ankara's message to allies, he argued, is simple: "Leave us alone in the Black Sea, we will handle this."

The 2022 test: Article 19 and the fleet that stayed outside

Days after the invasion began, Turkey invoked Article 19. Neither Russia nor Ukraine could send new warships through the straits, though vessels registered to Black Sea home ports could return to base. De Waal said the effect was immediate: "Up to 30 Russian ships stayed outside." That denial of reinforcement, he argued, "contributed to Ukraine holding the upper hand in the Black Sea." The convention, designed for an era of battleships, had inadvertently constrained a modern great-power fleet.

Ukraine adapted. "Ukraine didn't build military vessels," said Romanian analyst Iulian Chifu. "It just shifted the generation of attack." Sea drones and missiles, unregulated by a 1936 text, have sunk or damaged a significant portion of Russia's Black Sea Fleet, forcing its withdrawal from Sevastopol to Novorossiysk. The treaty did not anticipate unmanned warfare; it simply blocked the conventional response.

NATO's minesweepers stuck in Portsmouth

The treaty also binds Ukraine's supporters. In 2024 the United Kingdom donated two minesweepers to Kyiv. They never reached the Black Sea. Turkey classifies them as vessels of war, and the UK Ministry of Defence confirmed they remain docked in Portsmouth. The episode illustrates the hard limit Montreux places on Western military aid: even non-combatant naval assets cannot enter if Ankara decides they fall under the convention's definition of a warship.

Romania and Bulgaria: the NATO alternative

With the straits closed to allied warships, NATO has shifted focus to its Black Sea members. At the alliance's summit in Ankara earlier this month, Turkey, Romania and Bulgaria announced a trilateral task group for mine-clearing to protect underwater infrastructure. But Chifu was sceptical: "The military capabilities of Bulgaria and Romania are barely limited to their capacity of defending their territories." Romania is trying to change that. Its government plans to spend €2.5 billion ($2.8 billion) expanding Mihail Kogalniceanu Air Base near Constanța into the largest NATO base in Europe, surpassing Ramstein in Germany.

The investment signals Bucharest's ambition to become the alliance's primary Black Sea hub. Yet the base's utility depends on the straits remaining open to NATO aircraft and supply ships, a permission Turkey controls. "You have no option but to work with Turkey," de Waal said. "You can't work without it."

What happens when the war ends

Chifu, who describes himself as "not revisionist," believes the convention should hold while the war rages. "But after the war," he said, "we should reconsider very clearly how this is applied to our allies in NATO." The question is whether a post-war settlement will reopen Montreux. Russia accepts the current regime grudgingly; Turkey treats it as existential. Any renegotiation would require Moscow's agreement, the very Pandora's box de Waal warned about.