Since spring 2022, Russia has been prevented from reinforcing its navy in the Black Sea by a near-antique international convention and some very contemporary Turkish politics. A ceasefire could change that.
After more than three years of war in Ukraine, the election of Donald Trump as president of the United States has changed the game. On May 11, Ukrainian President Volodymyr Zelensky accepted a Russian proposal for direct talks in Istanbul, challenging Vladimir Putin to show up in person and agree to a thirty-day ceasefire. The Turkey meeting may or may not happen, and the guns may or may not fall silent for some period of time — but the war is now fought as much on the diplomatic chessboard as on the battlefield.
Although a durable peace remains improbable, the conflict could conceivably shift into a new normal, in which fighting is interspersed with talks and truces, and where both sides exploit pauses to rearm.
Interestingly, Turkey isn’t just a facilitator of the emerging diplomacy, but also an influential actor in its own right. If some form of sustained de-escalation were to occur, the area in which new force build-ups could happen most rapidly may be the Black Sea, where Turkey has since 2022 barred warships from entering, citing the Montreux Convention.
Policymakers now need to prepare for the possibility that Ankara would react to a longer-term ceasefire by lifting those restrictions, allowing the Russian Navy to surge into the Black Sea — and potentially setting the stage for future escalation.
The Montreux Convention
Every visitor to Istanbul will have witnessed the elephantine procession of cargo ships and tankers traversing the Bosporus, on their way in or out of the Black Sea. Farther south, ships ply the long and narrow Dardanelles, which opens on the Mediterranean. Together, these straits form the only way in or out of the Black Sea.
The Turkish Straits are highly strategic, serving as a major chokepoint for global oil and food shipments, including a fifth of the world’s wheat.
As noted by the late British expert on Soviet naval affairs, Michael MccGwire, the Black Sea can, in times of war, transform into “a grenade in Russia’s gut.” This unforgiving geography has had the effect of forcing Moscow to look to the Straits as a first line of defense, creating a permanent point of tension between Russia and Turkey and ensuring that the so-called Straits Question lingered as a constant feature of 19th- and 20th-century great-power politics.
Following the Ottoman Empire’s dismemberment after World War I, the Straits came under international control, before reverting to the modern Republic of Turkey. Since 1936, Turkey has managed the Straits under rules laid down in the Montreux Convention, which mandates free transit for all merchant shipping while limiting external powers’ naval access to the Black Sea. Remarkably, the convention remains in effect and unamended after nine decades, having survived not only World War II but also the Cold War and the constant commotion of Turkish politics.
Parsing the Convention
Although it has generated its share of arguments over the decades, the Montreux Convention remains a short and accessible document. The rules for naval transit are found in Section II, which includes Articles 8–22, and in Annexes II, III, and IV. The basic principle is that different states face different degrees of regulation depending on whether they are Turkey, another Black Sea littoral state, or one that is external to the region. Furthermore, these rules change in times of war.
Turkey’s own naval movements are entirely unrestricted in all circumstances. The other Black Sea states (currently Bulgaria, Romania, Ukraine, Russia, and Georgia) face some restrictions, mostly related to advance notice and the aggregate tonnage and number of warships allowed to transit at any one time. Non-Black Sea states face much stricter limitations, including a 21-day cap on warship visits past the Straits. The combined tonnage of visiting warships from non-littoral nations must also remain at all times below 45,000 tons — or 30,000 tons for a single nation, and 15,000 tons for a single warship. Both Black Sea and non-Black Sea states are banned from sending submarines and aircraft carriers through the Straits, although submarines belonging to the Black Sea powers may exit for repairs and then return. Over the decades, Moscow has manipulated these rules by sending submarines to Saint Petersburg for repairs and then letting them linger for extended periods in the Mediterranean, and by insisting that its carriers are in fact not carriers, but aircraft-carrying cruisers.
In sum, the Montreux Convention makes it very difficult for non-regional states to project naval power into the Black Sea. Their navies cannot base ships in the Black Sea and they are unable to make surprise appearances, deploy many large surface combatants at once, or send carriers, submarines, and large amphibious ships past the Straits. In practice, these rules leave Russia and Turkey as the two clearly dominant naval powers of the Black Sea.
In times of war, special regulations kick in. Those rules are detailed across Articles 19, 20, and 21.
The latter two articles deal with conflicts in which Turkey is a belligerent, or where it fears that it is about to be attacked imminently. Article 20 states that if Turkey is a party to the conflict, all rules for warship transit go overboard, and Ankara assumes sole control. Article 21 lets Ankara claim that full control preemptively if faced with imminent attack — but it must then notify other signatories and the League of Nations, which can overrule Turkey’s decision. Here, the convention shows its age. Vast geopolitical changes have occurred since 1936, including the disappearance of the Soviet Union and the League of Nations. As a result, any invocation of Article 21 would likely be mired in legal and diplomatic challenges.
Article 19, finally, regulates wars of a more common kind — namely those in which Turkey is not a belligerent and does not dread impending attack. The gist of it is that all non-belligerent powers will retain the same rights and obligations as in peacetime, while all belligerent powers are fully banned from sending warships through the Straits. There are rare exceptions, such as if a warship is trapped by conflict on the wrong side of the Straits — it is then allowed to return home expeditiously.
The wartime rules in Articles 19, 20, and 21 have proven difficult to disassociate from political considerations. In part, that is because of a curious omission in the Montreux Convention: Even though so much hinges on whether a state of war is present or not, the convention offers no definition of “war.”
In practice, therefore, it is solely up to Turkey to decide when to apply Article 19, with no one able to overrule its decision.
The Convention and the Russo-Ukrainian War
When Russia first invaded Ukraine in early 2014, Turkey protested and rejected Russia’s annexation of Crimea. But with an eye on the Montreux Convention, Ankara also refrained from labeling that conflict a war. Had it done so, it would have been required under Article 19 to ban transit by the Russian and Ukrainian navies.
At the time, Turkey’s non-invocation of Article 19 was made easier by the stealthy nature of the initial attack, specifically Russia’s use of Ukrainian proxy forces, and by the limited scope of the conflict.
Things were markedly different in February 2022, when Russia launched an all-out invasion of Ukraine. From the outset, Russian leaders refused to admit to a state of war, speaking euphemistically instead about their “special military operation.” If that was an attempt to encourage Turkey to sidestep Article 19, it failed. After some initial hesitation, Foreign Minister Mevlüt Çavuşoğlu announced on February 27, 2022, that Turkey considered the situation in Ukraine a war and would implement the Montreux Convention accordingly.
In fact, Çavuşoğlu went even further by saying that Turkey “had warned all riparian and non-riparian countries not to let warships go through the straits.” In other words, Turkey discouraged all naval transit through the Straits, regardless of which belligerent or non-belligerent, Black Sea or non-Black Sea nations were involved. There is no basis whatsoever for that kind of all-encompassing ban in the Montreux Convention, and Çavuşoğlu’s declaration gave rise to much speculation.
Some have argued that Turkey must now be utilizing Article 21, as only unrestricted Turkish control could allow Ankara to issue a complete ban on naval transit. It is an unpersuasive interpretation, as Turkey has never once claimed to fear an impending attack — nor is there any sign that it has issued the multiple notifications required by Article 21.
A more likely explanation is that Çavuşoğlu engaged in a face-saving sleight of hand, doing two things at once while pretending they were the same.
On the one hand, Turkey properly applied the Montreux Convention by banning Ukrainian and Russian naval transit, in accordance with Article 19. On the other hand, it tried masking the limited extent of Article 19 by warning off other navies, too — presumably to reduce the risk of Black Sea escalation and to sweeten the deal for Russia. Çavuşoğlu’s warning was, nevertheless, merely a plea dressed up as a decision, and the Montreux Convention does not permit Turkey to enforce it. Other NATO members could at any time have called Ankara’s bluff by declaring their intent to send a warship through the Straits, and Turkey would then have had to eat its words and let them through — or lunge for a spurious last-resort invocation of Article 21, at the risk of discrediting the convention.
To date, Turkey’s allies have not done so. Most likely, they concluded that a few more NATO vessels in the Black Sea could not warrant the troubles provoked by forcing Turkey’s hand. As a result, Çavuşoğlu’s warning remains unchallenged.
Dealing with a Deal
A short, fleeting cessation of violence might not mean much for the status quo in the Black Sea, but a longer-term ceasefire or a sustained de-escalation in Ukraine would likely tempt Turkey to reverse its closure of the Straits.
There is some precedent for such a move. A period of less intense, stop-and-start fighting might more closely resemble the early phases of the Russo-Ukrainian conflict after 2014 than the unabated full-scale war that has prevailed since 2022. As noted above, Turkey chose not to categorize the 2014–22 conflict as a “war” in the context of the Montreux Convention.
When trying to determine the continued applicability of Article 19, Turkish policymakers have strong incentives to take even weak and wobbly ceasefire agreements at face value. If they set the bar too high, they could end up trapping themselves in an extended wait for a clean and comprehensive agreement — which may not arrive for years, or ever. By instead moving swiftly to end restrictions at the onset of any somewhat serious ceasefire, Ankara gains flexibility and leverage over Russia, which must reckon with its ability to shut the Straits again as a potential consequence of renewed escalation.
But here’s the issue: Even a brief reopening of the Straits could be enough to transform the balance of power in the Black Sea.
Unlike men in trenches, warships are quick and easy to move, and unlike Ukraine, Russia has a large navy spread over many seas. If transit restrictions were lifted, Russia could rapidly transfer assets from its Northern, Baltic, or Pacific fleets to the Black Sea Fleet. For example, it might add more warships with long-range air defense systems, or cruise-missile carrying submarines capable of striking targets on land. Russia could also attempt to rebuild a force capable of interfering with Ukraine’s grain exports while protecting its own grain carriers against retaliation, and then leverage that imbalance as soon as the war resumes.
The Ukrainian Navy has no comparable extra-regional assets to call on. If Turkey were to lift restrictions, Ukraine would finally be able to collect four minehunters gifted by the United Kingdom and the Netherlands, which remain outside the Black Sea — but that is all. Transferring additional ships to the Ukrainian Navy would take time, including the time needed to train crews.
Of course, NATO members with powerful navies would once again be able to send their own warships into the Black Sea. But while such deployments would be most unwelcome to Russia, they would remain subject to the usual Montreux Convention restrictions, preventing them from lingering too long and forcing them to leave without replacement if the war resumes.
Despite these advantages, Russia may still decide against a naval surge. For one thing, it needs to consider the fact that Turkey could close the Straits again if clashes resume, forcing newly arrived ships either to leave immediately or be trapped. More importantly, Ukraine has turned a weak hand into an advantage in the Black Sea since 2022, countering Russia’s vast naval superiority through the use of shore-based missiles, unmanned systems, and innovative tactics. As a consequence of Ukraine’s relentless attacks, the Black Sea Fleet has been forced to relocate from its longstanding Crimean headquarters of Sevastopol to Novorossiysk, Russia — and Ukraine has kept striking there, too. After suffering through this three-year walloping, Moscow may not feel any great urge to line up more targets for Ukraine in the Black Sea.
Engaging Turkey
By itself, one short ceasefire may not make Turkey change its approach to the Montreux Convention. But if negotiations and truces gradually become the norm and the conflict shifts toward an indefinite 2014–22 type blend of fighting, talking, and oft-broken agreements, Ankara will likely reopen the Straits sooner or later. When it does, that decision could add new volatility to the war by enabling a rapid expansion of Russia’s naval power in the Black Sea.
Having failed to prepare for the Trump administration’s reversal of U.S. policy in early 2025, Ukraine’s friends in Europe must not drop the ball again.
As ceasefire talks take center stage, European nations should prepare for the high risk that brief ceasefires will breed flare-ups of intensified conflict. In so doing, they should consider the maritime dimension and the effects of a de-escalation on Turkey’s interpretation of the Montreux Convention. They should privately and publicly push for clarity on Turkey’s benchmarks for applying Article 19, urging Ankara to act consistently and predictably, while also conveying their own understanding of the convention.
Meanwhile, it is time to figure out how to best help Kyiv ensure that any new naval escalation by Russia in the Black Sea ends like the first one did — shipwrecked against Ukraine’s grit and ingenuity.
Aron Lund is a senior analyst at the Swedish Defence Research Agency, where he studies the politics and security of the Middle East, North Africa, and the Eastern Mediterranean. He is also a fellow at Century International and the Swedish Institute of International Affairs.
Image: Petty Officer 2nd Class Ford Williams
