The Montreux Convention: Turkey's 90-Year Gatekeeper Role in the Black Sea Security Crisis

Today, more than four years after Russia's full-scale invasion of Ukraine, the Black Sea has become one of Europe's key security frontiers. Yet it is "quite empty of warships," Thomas de Waal, a senior fellow at Carnegie Europe, told DW. "This is a bit of a paradox."

The Black Sea borders six countries: Turkey, Bulgaria, Romania, Ukraine, Russia, and Georgia. The first three are NATO members; the other three are not. Two of the non-NATO states—Russia and Ukraine—are at war with each other.

Access to the Black Sea runs through a single, narrow bottleneck: the Dardanelles Straits and the Bosporus, which crosses through Istanbul, Turkey's largest metropolis. At its narrowest point, the Bosporus is just 700 meters wide. For comparison, the Strait of Hormuz is approximately 33 kilometers wide at its narrowest point.

This geographic bottleneck makes the Black Sea "an economic choke point," de Waal said. "Whoever controls the Turkish Straits also controls, to some extent, trade through the Black Sea and also has an enormous military advantage."

Russia has a special interest in the Black Sea. "A strong military position in the Black Sea for Russia is not an end in itself, but also a means to advance its geopolitical ambitions beyond and also strengthen its great power status," said Daria Isachenko, a visiting fellow at the German Institute for International and Security Affairs.

The second part of the equation goes back to a treaty signed exactly 90 years ago.

On July 20, 1936, representatives from just under a dozen countries gathered in Montreux, Switzerland, to establish the Montreux Convention. This agreement restored Turkey's sovereignty over the Dardanelles Strait and Bosporus after years of international oversight following the Ottoman Empire's WWI defeat. The nearly 30-page document set the legal framework governing the Turkish Straits to this day.

The treaty keeps the straits open to passenger and commercial traffic during peacetime. Black Sea states may move warships through relatively freely. Outside-region navies face restrictions on fleet size, tonnage, and duration—no longer than 21 days during peacetime.

During wartime, rules become stricter. When Turkey is not part of the conflict, warships belonging to any nation involved in the fight cannot pass through—a clause with profound consequences since February 2022.

"The most privileged Black Sea state is Türkiye itself," Isachenko said. This explains why NATO doesn't maintain continuous presence in the Black Sea. "For NATO, Montreux is a constraint," Isachenko pointed out. "For Türkiye, it is an instrument to keep the balance in the Black Sea."

When Russia launched its full-scale invasion of Ukraine in late February 2022, the Montreux Convention faced its greatest test. Days after the operation began, Turkey invoked Article 19, restricting military ships from belligerent countries. Neither Russia nor Ukraine could freely reinforce naval forces, except warships registered to Black Sea home ports could return to base.

"I think the Montreux Convention and Turkey's policy at the beginning of the war was absolutely crucial in allowing Ukraine to survive," de Waal told DW. It prevented Russia from adding vessels, with "up to 30 Russian ships stayed outside," contributing to Ukraine holding "the upper hand in the Black Sea."

These restrictions highlighted how warfare has changed. "Ukraine didn't build military vessels," said Iulian Chifu, a Romanian foreign policy analyst. "It just shifted the generation of attack."

The treaty also restricted Ukraine's Western supporters. In 2024, the UK donated two minesweepers unable to enter the Black Sea because Turkey considers them "vessels of war." Instead, they dock in Portsmouth, England.

"It's a very carefully constructed strategy," de Waal said, in which Turkey tells NATO countries: "Leave us alone in the Black Sea, we will handle this."

Turkey has created an arrangement Russia accepts despite displeasure. "Turkey has come up with an arrangement, a regime, which Russia is not especially happy about, but accepts, and that's very important," he noted.

The Montreux Convention stands as a rare example of a multilateral diplomatic mechanism still observed and respected 90 years after signing, even as the international rules-based order faces pressure.

The Turkish Straits' geography and complex governance led NATO to rethink operations. "In the Black Sea," de Waal explained, "Turkey is very much Turkey first and NATO second."

Turkey's message to Western allies: "We don't want to see your permanent presence in the Black Sea, but Romania and Bulgaria have this shared Black Sea ownership, so you can support them."

Greater focus has been placed on Romania and Bulgaria, NATO members since 2004. "The military capabilities of Bulgaria and Romania," said Chifu, "are barely limited to their capacity of defending their territories."

Nevertheless, Romania has been "trying to improve our capabilities building ships," purchasing military assets to "enforce our footprint in the Black Sea." One example is the planned expansion of Mihail Kogalniceanu Air Base near Constanța. Romania expects to spend €2.5 billion ($2.8 billion) transforming it into Europe's largest NATO military base—bigger than Ramstein in Germany.

Earlier this month, NATO leaders met in Ankara for their annual summit. Turkey, Romania, and Bulgaria announced a trilateral task group for mine-clearing efforts to protect underwater infrastructure.

The Montreux Convention is "quite an archaic document," de Waal said, still referring to the League of Nations. "But I think no one wants to touch it," out of concern it "opens a kind of Pandora's box when all the issues have to be renegotiated with Russia."

For Türkiye, Montreux "belongs to a number of founding treaties of the Turkish Republic," reflecting core interests: "sovereignty, security and status," Isachenko said. Challenging it means "dealing, revising and challenging" those interests.

Chifu was "not revisionist" and believed the convention should stay while the war rages. "But after the war," he said, "we should reconsider very clearly how this is applied to our allies in NATO."

"You have no option but to work with Turkey," de Waal took from the Ankara summit. "You can't work without it," referring to the strongest power in the Black Sea.

For nine decades, Montreux has outlasted wars, leaders, and generations. As long as it lasts, Turkey holds sway over the Black Sea gateway.

Editors: Peter Hille and Don MacCoitir