PRE WWII

An 87-year-old deal is keeping a lid on the naval war in Ukraine — for now

Constantine Atlamazoglou
Updated Thu, October 19, 2023 


Fighting between Russia and Ukraine in the Black Sea has picked up in recent months.


The fighting is taking a toll on Russia's Black Sea Fleet, which Moscow can't reinforce.


Turkey's enforcement of the Montreux Convention limits what Russia and NATO can send into the sea.

While Ukrainian and Russian forces are locked in a grinding battle for territory in eastern and southern Ukraine, fighting in the Black Sea has picked up.

Ukraine has conducted several high-profile attacks against Russian targets in recent weeks, including missile strikes on occupied Crimea that hit Russia's Black Sea Fleet headquarters and Russian navy dry docks. Its strike on the dry docks took out a landing ship and a Kilo-class submarine and is set to further hamstring Russian naval logistics.

Those attacks were not Kyiv's first successes against Russia's navy, but Ukraine lacks a fleet — it scuttled its flagship in March 2022 to prevent its capture by Russia — and has relied on asymmetrical warfare, including naval drones, to counter Russia in the Black Sea.

Russia's Black Sea Fleet hasn't been defeated, but its losses, including the sinking of its flagship, the Moskva, weigh more heavily on Russia's war effort because of a deal signed nearly a century ago that's preventing Moscow from bringing more ships into the Black Sea.

Ships bottled up

Russian warships at the Black Sea Fleet base in Sevastopol in July 2010.AP Photo/Sergey Ponomarev

Signed in 1936, the Montreux Convention governs the transit of merchant vessels and warships through the Dardanelles and the Bosporus Strait and the presence of warships in the Black Sea.

The convention distinguishes between Black Sea powers — those with a Black Sea coastline — and non-Black Sea powers. In peacetime, warships belonging to non-Black Sea powers cannot stay in the sea for more than 21 days.

The convention also has limits for the combined tonnage of vessels from non-Black Sea powers that can be in the sea at one time, capping it at a maximum of 45,000 tons with no more than 30,000 tons belonging to one country. A US Navy Arleigh Burke-class destroyer, several of which have sailed into the sea in recent years, is about 9,000 tons.

While the convention doesn't specifically rule out aircraft carriers, a 15,000-ton limit for vessels of non-Black Sea powers transiting the straits effectively prohibits them. Submarines from non-Black Sea powers are also not allowed.

Russian Navy submarine Rostov-on-Don sailing through the Bosphorus to the Black Sea in February 2022.OZAN KOSE/AFP via Getty Images

Turkey controls the straits connecting the sea to the Mediterranean, and all foreign warships have to notify Turkey prior to passage — 15 days ahead of time for non-Black Sea powers and eight days for Black Sea countries.

During war, the convention permits Turkey to limit the passage of warships to the Black Sea — even if Turkey is not at war — unless these ships are returning to their base. On February 28, 2022, four days after Russia attacked Ukraine, Turkey invoked that power.

"When Turkey is not a belligerent in the conflict, it has the authority to restrict the passage of the warring states' warships across the straits. If the warship is returning to its base in the Black Sea, the passage is not closed," Turkey's foreign minister said at the time. "All governments, riparian and non-riparian, were warned not to send warships across the straits."

Ankara's invocation of the convention is seen as significant because it has prevented Russia from bolstering its Black Sea Fleet. If Turkey were to open the straits, "the first thing you would see is a significant naval reinforcement from other parts of the Russian fleet, and that isn't in Ukraine's interest or ours," Ben Wallace, Britain's defense minister at the time, said at the Shangri-La Dialogue in June.

Smoke rising from a fuel tank after an attack in Sevastopol in April.REUTERS/Stringer

While Turkey's move has limited Russian naval movements, experts differ on its overall impact on the war.

"Initially, it was thought that it was a big deal that some of the Russian warships were prevented from entering the Black Sea. But Russia had a massive naval presence there anyway," including six ships from its Baltic and Northern fleets that entered the sea before Turkey invoked the convention, Volodymyr Dubovyk, a professor at Odessa's I. Mechnikov National University, told Insider.

Dubovyk added that Russia's ships in the Black Sea didn't give it a "decisive edge over Ukraine," and because of the effectiveness of Ukraine's attacks, those ships had become "sitting ducks" that had "to hide away from the Ukrainian coast."

Russian warships still appear to be stationed at Sevastopol, the main Russian naval base in Crimea, but Ukraine's success in targeting that base and Russian forces nearby will probably have a lasting impact.

"I think that these developments are going to contribute to a change in thinking about the role of naval power," Dubovyk told Insider.

Allied help

A Turkish navy frigate alongside a Romanian frigate in the Black Sea in February 2018.NATO

Iulia-Sabina Joja, the director of the Black Sea program at the Middle East Institute, a think tank in Washington, DC, said the convention also allowed Turkey to shape the international response to the war.

"The Montreux Convention is confusing," Joja told Insider. "It actually states that when Turkey is not a belligerent but feels threatened, then it can act as if it were a belligerent."

In such a case, as Joja noted, the decision about which warships can enter the Black Sea is entirely up to the Turkish government, according to the convention. (Merchant ships can pass through the straits freely, even during wartime, and satellite imagery suggests Russia is using them to move military hardware through the sea.)

This provision could allow Turkey to let warships from NATO navies into the Black Sea, and some experts have advocated that the alliance seek access to the sea to escort merchant vessels carrying Ukrainian grain, thus helping circumvent Russia's blockade.

A cargo ship passes a beach on the Danube River, which is being used for Ukrainian grain shipments, in Izmail in August.Getty Images

While the convention may allow such an action, going through with it would be politically difficult for NATO and for Ankara.

"We have two unknowns here: whether the Turkish government is willing to let NATO maritime defense capabilities in and whether NATO allies have the political will to build a credible deterrence and defense in the Black Sea region," Joja told Insider.

Turkey is a NATO member but maintains a good relationship with Russia. That has, at times, frustrated other NATO members, but Ankara used it to help broker the 2022 deal allowing Ukraine to ship grain through the Black Sea. (Russia withdrew from the deal in July and imposed its blockade.)

Allowing NATO warships into the Black Sea now would test Turkey's relations with Russia, but, Joja said, it would also benefit the alliance, "contribute to security in the Black Sea region, and would indirectly even help Ukraine ship grain out of the Black Sea."

Constantine Atlamazoglou works on transatlantic and European security. He holds a master's degree in security studies and European affairs from the Fletcher School of Law and Diplomacy. You can contact him on LinkedIn and follow him on X.