Why a 90-year-old treaty limits NATO in the Black Sea

Now more than four years after Russia’s full-scale invasion of Ukraine, the Black Sea has become one of Europe’s key security frontiers. And yet, it is currently “quite empty of warships,” Thomas de Waal, a senior fellow at Carnegie Europe, told DW. This, he said, “is a bit of a paradox.” To understand that paradox, we first need to need to take a closer look at the map.

The Black Sea is bordered by six countries: Turkey, Bulgaria, Romania, Ukraine, Russia, and Georgia. The first three on that list are part of the North Atlantic Treaty Organization, or NATO — the other three are not. To make matters more complicated, two of the latter three are at war with each other.

Narrow bottleneck to maneuver

Access to the Black Sea runs through a single, narrow bottleneck. To get there, a ship coming from the Mediterranean Sea would have to pass the Dardanelles Straits and the Bosporus, which crosses through the heart of Turkey’s largest metropolis Istanbul.

At its narrowest point, the Bosporus is just 700 meters (0.4 miles) wide. For comparison, another critical passageway currently in the headlines is the Strait of Hormuz, which is approximately 33 kilometers (21 miles) wide at its narrowest point.

It is this geographic bottleneck to the Black Sea that makes it “an economic choke point,” said de Waal. “Whoever controls the Turkish Straits also controls, to some extent, trade through the Black Sea and also has an enormous military advantage.”

Russia also has a special interest in the Black Sea, according to Daria Isachenko, a visiting fellow at the German Institute for International and Security Affairs. “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.”

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

Treaty that made Turkey gatekeeper

On July 20, 1936, representatives from just under a dozen countries gathered in the Swiss town of Montreux to settle a long-standing dilemma: How — and by whom — should these vital waterways be governed? They established the Montreux Convention, an agreement that essentially restored Turkey’s sovereignty over the Dardanelles Strait and Bosporus. This came after years of international oversight, following the Ottoman Empire’s defeat in the First World War. 

Negotiations resulted in a document of nearly 30 pages, which set the legal framework that governs the Turkish Straits to this day. The treaty keeps the straits open to passenger and commercial traffic during peacetime — crucial for the important trade route. 

When it comes to military ships, Black Sea states are allowed to move their warships through the straits relatively freely. Any navy from outside the region faces restrictions on their fleet size, tonnage, and how long they are allowed to stay in the sea — no longer than 21 days during peacetime.

Special rules for war

During times of war, these rules become stricter. When Turkey is not part of the conflict, warships belonging to any nation involved in the fight are not allowed to pass through — a clause that has had profound consequences for the region since February 2022.

This special system of access rights keeps the Turkish government in a very powerful position. “The most privileged Black Sea state is Türkiye itself,” Isachenko said.

It also helps explain why NATO doesn’t maintain a continuous presence in the Black Sea — unlike in other waters such as the Mediterranean or the North Sea. “For NATO, Montreux is a constraint,” Isachenko pointed out. “For Türkiye, it is an instrument to keep the balance in the Black Sea.”  

War in Ukraine tests Montreux Convention

When Russia launched its full-scale invasion of Ukraine in late February 2022, the Montreux Convention arguably faced its greatest test in decades. A few days after the military operation began, Turkey invoked Article 19 of the treaty, which applies during wartime and restricts military ships belonging to countries engaged in the fight from passing through the straits.

Essentially, from that point onwards, neither Russia, nor Ukraine, could freely reinforce their naval forces in the Black Sea — with one exception: warships registered to a home port in the Black Sea could return to their base.

“I think the Montreux Convention and Turkey’s policy at the beginning of the war was absolutely crucial in allowing Ukraine to survive,” Thomas de Waal told DW.

He argued that it prevented Russia from adding more vessels to their Black Sea fleet, saying that “up to 30 Russian ships stayed outside.” This, he added, contributed to Ukraine holding “the upper hand in the Black Sea.” 

Those restrictions may also have highlighted how warfare itself has changed, with Ukraine turning to drones and other modern technology not covered by the 1936 document. “Ukraine didn’t build military vessels,” according to Iulian Chifu, a Romanian foreign policy analyst. “It just shifted the generation of attack,” he said.

The maritime treaty has also restricted the options available to Ukraine’s supporters in the West. In 2024, the United Kingdom donated two minesweepers to Ukraine, which were unable to enter the Black Sea, according to the UK Ministry of Defence. That’s because Turkey considers them to be “vessels of war,” according to de Waal. Instead, the ships have been docked in Portsmouth, in southern England.

For de Waal, this illustrates Turkey’s wider approach: “It’s a very carefully constructed strategy,” he said,” in which Turkey basically says to other NATO countries: ‘Leave us alone in the Black Sea, we will handle this.'”

Key to the convention’s enduring relevance may be its long-standing recognition by states. “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.

At a time when the international rules-based order has increasingly come under pressure, the Montreux Convention stands out as a rare example of a multilateral diplomatic mechanism that is still observed and respected 90 years after it was signed. 

How is NATO adapting?

The geography of the Turkish Straits — plus their complex governance structure — have led to NATO rethinking how it can operate in the region. At times, the issue even creates some tension between its alliance members: “In the Black Sea,” de Waal explained, “Turkey is very much Turkey first and NATO second.”

This security logic, he argued, has shaped Turkey’s message to its 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.”

As a result, greater focus has been placed on these two Eastern European states, which have been NATO members since 2004. However, some experts remain doubtful: “The military capabilities of Bulgaria and Romania,” said Chifu, “are barely limited to their capacity of defending their territories.”

Nevertheless, he argued that Romania had been “trying to improve our capabilities building ships,” as well as purchasing military assets, with the aim of “enforcing our footprint in the Black Sea.”

One example is the planned expansion of Mihail Kogalniceanu Air Base near the Romanian port city Constanța. Romania’s government is expected to spend € 2.5 billion ($ 2.8 billion) on transforming it into the largest NATO military base in Europe — bigger than Ramstein, a US military base in Germany. 

Earlier this month, the region was in the spotlight once again, as leaders of NATO’s member states met in the Turkish capital Ankara for their annual summit. There, Turkey, Romania, and Bulgaria announced a trilateral task group for mine-clearing efforts to protect underwater infrastructure.

What is next for the 90-year-old treaty?

The Montreux Convention is “quite an archaic document,” de Waal told DW, pointing out that it still refers to the League of Nations — the UN’s predecessor, which ceased to exist 80 years ago. “But I think no one wants to touch it,” out of concern that it “opens a kind of Pandora’s box when all the issues have to be renegotiated with Russia,” he added.

“For Türkiye,” said Isachenko, Montreux “also belongs to a number of founding treaties of the Turkish Republic.” Beyond its role as a maritime access treaty, it reflects Turkey’s core interests: “sovereignty, security and status,” she added, so challenging Montreux means “dealing, revising and challenging” those core interests.

Security analyst Chifu stressed that he was “not revisionist” and believed the convention should stay in place while the Ukraine war rages on. “But after the war,” he said, “we should reconsider very clearly how this is applied to our allies in NATO.” 

To him, the question of how exactly the treaty’s clauses are applied and interpreted by Turkey remains a sore point. 

“You have no option but to work with Turkey,” is the key message de Waal took away from the recent NATO summit in Ankara. “You can’t work without it,” he said, referring to what he calls the strongest power in the Black Sea.

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

Editors: Peter Hille and Don MacCoitir