The Montreux Convention warship rules give Turkey something no other NATO member possesses: legal authority to close a strategic waterway to foreign navies in wartime. Signed in 1936, the convention governs passage through the Bosphorus Strait and the Dardanelles Strait, the only maritime route between the Mediterranean and the Black Sea. Its core mechanism is a tiered Straits Passage Regime that treats Black Sea littoral states differently from non-littoral powers, restricts warship tonnage and duration, and grants Turkey the right to restrict belligerent warships when it is not itself at war. In practice, this means Ankara decides which warships enter the Black Sea and which do not, a power it has exercised most visibly since Russia's full-scale invasion of Ukraine in February 2022.
The convention is often described as a Cold War relic, but its design reflects a specific historical compromise: Turkey wanted sovereignty over the straits, the Soviet Union wanted to keep non-Black Sea navies out, and Britain wanted to preserve access for its own fleet. The resulting text is a bundle of compromises that has proven remarkably durable. It has governed warship transits for nearly nine decades, through the Cold War, the collapse of the Soviet Union, and two major Black Sea conflicts. Understanding the Montreux Convention warship rules requires separating what the text actually says from what states claim it means, because the gap between the two has become the central battleground of Black Sea geopolitics.
The Tiered Tonnage System
What Non-Littoral Navies Can Send
The convention's most consequential provisions for non-Black Sea states are its tonnage and duration limits. Non-littoral warships in transit through the straits are capped at nine vessels and 15,000 tons aggregate at any one time . Once in the Black Sea, the total tonnage that all non-littoral states combined may maintain is 45,000 tons, and any single non-littoral state is limited to 30,000 tons . Non-littoral warships may remain in the Black Sea for no more than 21 days . These limits were designed to prevent a hostile fleet from establishing a sustained presence in a sea that the Soviet Union considered its southern flank.
Certain categories of warship are barred entirely. Aircraft carriers may not transit regardless of which state they belong to . Non-Black Sea submarines are prohibited, while littoral submarines may pass only for construction, purchase, or repair purposes and must travel on the surface by day . These prohibitions reflect the naval technologies of the 1930s, when carriers and submarines were understood as the primary instruments of power projection. The categories have aged poorly: modern navies operate vessels that blur the lines between traditional classifications, creating interpretive disputes that Turkey must adjudicate as the convention's depositary and gatekeeper.
What Black Sea States Can Send
Black Sea littoral states—Turkey, Russia, Ukraine, Romania, Bulgaria, and Georgia—enjoy more generous rights. Their light surface vessels face the same 15,000-ton transit cap, but they may send capital ships exceeding that limit if the ships pass singly and are escorted by no more than two destroyers . Littoral submarines may transit for construction or repair, subject to daytime surface passage. Notification requirements are shorter: eight days for littoral states versus fifteen for non-littoral states . The asymmetry is intentional. The convention's drafters assumed that Black Sea states had legitimate interests in the sea that outside powers did not, and the tiered regime encodes that assumption in law.
The tiered system creates a structural advantage for Russia, which maintains the largest Black Sea fleet among the littoral states. Even when Turkey restricts belligerent transits under Article 19, Russian warships returning to their Black Sea bases are exempt, a carve-out that has allowed Russia to rotate vessels between the Mediterranean and the Black Sea throughout the Ukraine war . Ukraine, with a vastly smaller navy, benefits from the same legal framework in theory but cannot exploit it in practice. The convention's neutrality is procedural, not substantive: it treats all littoral states alike while the distribution of naval power among them is profoundly unequal.
Turkey's Wartime Powers
The Article 19 Trigger
Article 19 is the provision that transforms Turkey from a traffic regulator into a geopolitical actor. When Turkey is not a belligerent but a war exists involving other states, Turkey may prohibit the transit of warships belonging to the belligerent powers, except for vessels returning to their home ports in the Black Sea . This is the legal basis Turkey invoked on February 28, 2022, when it announced restrictions on Russian and Ukrainian warships following Russia's invasion . The exemption for returning vessels meant Russian ships already in the Black Sea could stay, but reinforcements from other fleets—such as the Slava-class cruisers Marshal Ustinov and Varyag—were barred from entering .
Turkey's application of Article 19 has been criticized from multiple directions. Ukraine and some NATO allies have argued that Turkey should do more to protect Ukrainian shipping and allow allied minesweepers to assist demining operations . Russia has warned that it expects Turkey to enforce the convention symmetrically, keeping non-littoral NATO navies out as rigorously as it keeps Russian reinforcements out . Turkey's position, articulated repeatedly by Defense Minister Yaşar Güler, is that it is applying the convention strictly and will continue to do so as long as the war continues . Ankara frames its role as regional stabilizer: by closing the straits to belligerent reinforcements, it prevents the Black Sea from becoming a direct NATO-Russia theater.
The Imminent Threat Clause
Article 21 gives Turkey an even broader authority: if it considers itself threatened with imminent danger of war, it may apply the provisions of Article 20, which leaves the passage of foreign warships entirely to the Turkish government's discretion . This clause has never been formally invoked, but its existence shapes expectations. It means Turkey's legal authority over the straits is not limited to declared wars. A sufficiently serious crisis—a Russian attack on a Turkish vessel, a NATO request that Ankara deems provocative—could trigger restrictions beyond the Article 19 framework. The clause is a reminder that the convention is not a neutral legal code but an instrument of Turkish statecraft, designed to give Ankara leverage in exactly the kind of contingency the Black Sea now faces.
The discretion Article 21 confers is not unlimited. Turkey is bound by the convention's procedural requirements, and blatant favoritism toward one belligerent would undermine the legal framework's legitimacy. But the line between strict application and political interpretation is thin. When Turkey allowed the USS Mount Whitney to transit in 2008 despite questions about its tonnage compliance, and again in 2014, critics accused Ankara of bending the rules for an ally . Turkey's response has consistently been that it interprets the convention in good faith and that its decisions serve regional stability. Whether that defense is credible depends on whether one sees Turkey as a neutral gatekeeper or a NATO member with interests of its own.
The Straits Passage Regime in Practice
Notification and Verification
The Straits Passage Regime operates through a notification system administered by Turkish authorities. Warships must notify Turkey through diplomatic channels before transiting: eight days in advance for Black Sea states, fifteen for others . The notification includes the vessel's name, type, tonnage, and intended dates of passage. Turkey's Foreign Ministry reviews the notifications and can reject transits that violate the convention's limits. In practice, the system functions smoothly during peacetime, with Turkey approving nearly all compliant notifications. During wartime, the same system becomes a mechanism of exclusion: a warship that would be welcome in peacetime may be barred under Article 19, and the diplomatic notification becomes the moment at which Turkey exercises its discretion.
Verification is a persistent challenge. The convention's tonnage limits depend on accurate declarations, and Turkey has no independent means of weighing a warship at sea. The USS Mount Whitney episodes illustrate the problem: the ship's declared tonnage shifted depending on who was counting, and Turkey accepted figures that allowed the transit . In most cases, the stakes are low, and states have little incentive to cheat. But in a crisis, a state seeking to slip a warship through the straits has obvious reasons to understate its tonnage or misclassify its vessel. Turkey's ability to detect and deter such violations depends on intelligence and political will, not on any enforcement mechanism built into the convention.
The Russian Exemption
The most consequential feature of the wartime regime is the exemption for warships returning to their Black Sea home ports. This provision has allowed Russia to maintain its Black Sea Fleet's strength despite Turkey's closure of the straits to reinforcements. Russian warships based in Sevastopol or Novorossiysk that were deployed to the Mediterranean before February 2022 have been permitted to return, while ships from the Northern or Pacific Fleets have been barred . The distinction is legally sound: the convention's text explicitly exempts returning vessels, and Turkey has applied the exemption consistently. But the effect is asymmetric. Russia can rotate its Black Sea forces; Ukraine cannot receive allied warships under any circumstances, because no allied warship has a Black Sea home port.
This asymmetry has fueled Ukrainian frustration. Kyiv has argued that Turkey's strict application of the returning-vessel exemption effectively favors Russia, because Russia has warships to rotate and Ukraine does not . Turkey's response is that the convention is not a sanctions regime and that Ankara cannot rewrite its provisions to disadvantage one party. The dispute is unresolvable within the convention's framework: the text treats all littoral states equally, but the distribution of naval power among them is unequal, and no interpretation can change that. Turkey's role as gatekeeper is legally neutral and politically consequential, a combination that satisfies no one fully and yet has preserved the Black Sea from direct NATO-Russia confrontation.
What the Convention Does Not Cover
Merchant Vessels and the Shadow Fleet
The Montreux Convention's warship rules are detailed and enforceable, but its merchant vessel provisions are permissive by design. In peacetime, merchant ships of any flag enjoy complete freedom of transit, and this freedom continues in wartime when Turkey is not a belligerent, subject only to rules about aiding the enemy . This permissiveness has created a loophole that Russia has exploited. Ukraine has accused Russia of using civilian auxiliary vessels to move weapons and military equipment through the straits, and ship-tracking analysts have documented roll-on-roll-off civilian vessels engaged in suspicious activity . Turkey has said it monitors compliance and has stopped some suspicious transits, but the convention's merchant provisions give Ankara limited legal grounds to intervene. A warship can be turned back under Article 19; a cargo ship claiming civilian status is far harder to exclude.
Russia's so-called shadow fleet—tankers and cargo vessels operating outside Western sanctions—has compounded the problem. These ships transit the straits carrying oil and grain, some of it allegedly seized from occupied Ukrainian territory . Turkey treats them as merchant vessels entitled to passage under the convention, which they legally are. Ukraine argues that Turkey should interpret the convention's aiding-the-enemy provisions more aggressively, but Ankara has resisted, wary of establishing a precedent that could be turned against Turkish shipping elsewhere. The result is a legal regime that constrains warships tightly while allowing civilian vessels to move with minimal interference, a distinction that makes sense in the context of 1936 but leaves gaps in the context of 2026.
The Aircraft Carrier Ambiguity
The convention's prohibition on aircraft carriers transiting the straits has generated persistent interpretive disputes. The text was drafted when carriers were a distinct and recognizable category, but modern naval architecture has blurred the line between carriers, amphibious assault ships, and helicopter carriers. The Soviet Union exploited this ambiguity in the 1970s by classifying its Kiev-class carriers as aircraft-carrying cruisers and sending them through the straits, a maneuver that Turkey accepted despite Western objections . More recently, the question of whether large amphibious ships with flight decks count as carriers has arisen in the context of NATO exercises and Russian naval deployments. Turkey has handled these disputes case by case, issuing rulings that satisfy the letter of the convention while accommodating the interests of the states involved. The pattern is characteristic of the Montreux regime: legal categories that were precise in 1936 have become negotiable in practice, and Turkey's interpretive authority has expanded accordingly.
The carrier ambiguity matters beyond the Black Sea because it affects the credibility of the convention as a whole. If the rules can be stretched to accommodate a vessel that looks like a carrier but is called something else, the prohibition loses its deterrent effect. Turkey has managed to preserve the convention's authority by resisting the most blatant attempts to circumvent it—the Varyag, an unfinished Soviet carrier towed through the straits in 1998 without engines or weapons, was allowed precisely because it was not an operational warship . But the line between legitimate interpretation and effective evasion is thin, and each new vessel class tests it. The convention's durability owes much to Turkey's judgment in navigating these disputes, a judgment that is political rather than legal and therefore impossible to codify.
Why the Convention Endures
The Balance of Dissatisfaction
The Montreux Convention survives because every party that matters is dissatisfied with it in ways that make revision risky. Turkey would like more discretion but benefits from the legitimacy the convention confers on its gatekeeping role. Russia would like unrestricted access for its Black Sea Fleet but relies on the convention to keep NATO navies out. The United States and NATO allies would like more access to the Black Sea but cannot propose an alternative that Turkey or Russia would accept. Ukraine would like the straits opened to allied warships but understands that any revision would require Russian consent, which would not be forthcoming. The convention's durability is a function of this balance: it is bad for everyone in some respects, which makes it acceptable to everyone in practice.
The Ukraine war has tested the convention more severely than any event since its signing. Turkey's closure of the straits to belligerent reinforcements has prevented the Black Sea from becoming a direct theater of NATO-Russia confrontation, a outcome that serves Turkish interests and arguably global stability. But the closure has also constrained Ukraine's ability to receive naval support and allowed Russia to maintain its fleet's presence. The convention has not resolved the Black Sea's strategic dilemmas; it has channeled them into legal procedures that Turkey manages. That is not nothing. In a region where great-power competition is intensifying, a legal framework that forces disputes into diplomatic channels rather than military ones has value that transcends its specific provisions.
The Succession Question
One question the convention does not answer is what happens when its signatories change. The Soviet Union signed the 1936 text; Russia claims to be its successor, and no state has formally challenged that claim. Ukraine and Georgia, which became independent after the Soviet collapse, are now Black Sea littoral states under the convention's terms, but their status as successors to Soviet rights and obligations was never formally negotiated. The convention's procedural provisions—notification requirements, tonnage limits, belligerent restrictions—apply to them as coastal states, and Turkey has treated them as such. But the deeper question of whether the convention's original balance of power assumptions still hold in a Black Sea with six littoral states, three of which are NATO members and one of which is at war with another, is unresolved.
Turkey's answer to this question has been pragmatic: apply the text as written, adapt interpretations to new circumstances, and avoid any move that would trigger a formal revision conference. This approach has preserved the convention through crises that might have destroyed a less flexible regime. But it has also deferred the harder questions indefinitely. At some point—after the Ukraine war ends, or when a new naval technology renders the tonnage limits obsolete, or when a littoral state demands a renegotiation—the convention will face a test it cannot pass through interpretation alone. Until then, the Montreux Convention warship rules will continue to shape the Black Sea's strategic geography, a 1936 text governing 2026 realities with surprising effectiveness and persistent tension.
The convention's endurance is a lesson in the value of imperfect institutions. It does not solve the Black Sea's problems; it manages them. It does not satisfy any party's maximal preferences; it establishes a baseline that all parties can live with. In a world where great-power competition is increasingly conducted through legal and procedural maneuvering, the Montreux Convention offers a model of how law can constrain power without pretending to eliminate it. Turkey's gatekeeping role is the convention's central mechanism and its central vulnerability: it works because Turkey is strong enough to enforce the rules and cautious enough not to abuse them. Whether that balance holds as the Black Sea's strategic environment evolves is the question on which the convention's future depends.