Published by The Lawfare Institute
in Cooperation With
One of the international legal consequences of Russia’s war of aggression against Ukraine is playing out in the Turkish Straits. Turkey is grappling with how to implement obligations under the 1936 Montreux Convention that require it to restrict transits by Russian warships. In doing so, Turkey could make a unique contribution to an array of global punishments rooted in international law. Instead, Turkey’s current chosen path of attempting to close the straits to all warships oversteps the Montreux Convention and risks replacing a long-standing set of rules vital to Turkish security with arbitrary restrictions. Rather than trying to prevent all warship transits, Turkey should close the Turkish Straits only to Russian and Ukrainian warships. This will uphold Turkey’s legal obligations and can serve its diplomatic interests.
The Montreux Convention
The Bosphorus and Dardanelles—together, the Turkish Straits—are the only path between the Mediterranean and Black seas. As a solution to the Straits Question that vexed Russian, Ottoman/Turkish, and British relations in the 19th and early 20th centuries, the Montreux Convention has governed navigation through the Turkish Straits since 1936, surviving both World War II and the Cold War. Under Article 24 of the convention, Turkey is charged with supervising “the execution of all the provisions … relating to the passage of vessels of war through the Straits.”
Under normal, peacetime rules, the convention regime guarantees the general right of warships of all states to transit the straits, but with a bias toward the six Black Sea riparian states (Georgia, Turkey, Bulgaria, Romania, Ukraine and Russia). To transit, warships of all other states must not have a displacement greater than 15,000 tons and may not stay in the Black Sea longer than 21 days. Black Sea states are exempt from those restrictions.
However, three articles modify the normal, peacetime rules safeguarding warships’ right to transit during times of war: Articles 19, 20 and 21.
Article 19 applies when a war exists and Turkey is not party to the conflict. At such times, “warships shall enjoy complete freedom of transit and navigation through the Straits” under the normal peacetime rules. However, warships of belligerent states “shall not … pass through the Straits” except if (a) one of the belligerents is acting under lawful collective defense rights obligations or (b) any belligerent warship must pass through the straits to return to its base.
Article 20 applies when Turkey itself is party to a war. In such cases, Turkey enjoys complete discretion over warship navigation through the straits, and no state, whether Black Sea or non-Black Sea, enjoys freedom of transit or navigation through the straits.
Article 21 applies a limited version of Article 20 powers when Turkey “considers itself to be threatened with imminent danger of war.” As in Article 20, the default, peacetime rules on warship navigation are suspended under Article 21. But unlike Article 20, under Article 21, ships separated from their bases by the straits must generally be allowed to transit to return home. Article 21 comes with an important procedural check. If Turkey invokes Article 21, it must notify other High Contracting Parties. If two-thirds of the “Council of the League of Nations” and half of the High Contracting Parties reject Turkey’s measures as unjustified, Turkey’s Article 21 invocation is suspended. If invoked today, some element of the United Nations, beginning with the General Assembly, might take up the issue in accordance with a 1946 U.N. resolution on the U.N.’s assumption of the League of Nations’ legal responsibilities.
In the current situation, Article 19 is the most defensible modification to the Montreux Convention’s warship navigation regime. Although Russia blocked a Security Council resolution condemning Russia for violating Article 2(4) of the U.N. Charter, Russia is plainly waging a conventional international war of aggression against Ukraine, and Turkey is not a party to the conflict. Turkey can therefore invoke Article 19 with respect to Russia and Ukraine.
Chronology of Events
Since Russia’s invasion of Ukraine began, Turkey’s response has swung from signaling an underapplication of the convention to signaling an overapplication of it.
On Feb. 24, the first day of the invasion, Ukraine asked Turkey to exercise its power under the convention to close the straits to Russian warships, and Ukrainian President Volodymyr Zelensky reiterated this plea in a tweet. Turkey’s initial response was noncommittal and emphasized that Russian warships would still transit.
On Feb. 27, Turkish Foreign Minister Mevlut Cavusoglu acknowledged Putin’s invasion as a war—an important first step toward invoking Article 19. (Faced with the Soviet Union insisting that Turkey apply Article 19 to U.S. warships during the Vietnam War, Turkey held that the U.S. was not at war.) A flurry of coverage predicted that an Article 19 invocation was imminent and the straits would be closed to Russian warships.
On Feb. 28, however, Turkey steered away from Article 19. Instead, Cavusoglu “warned all riparian and non-riparian countries not to let warships go through the straits” (emphasis added) without clarifying the basis for this warning. Although no implementing regulations have been published yet, Cavusoglu’s words suggest that Turkey would close the straits to all warships, not just those from Russia and Ukraine.
The Legality of Turkey’s Direction
Although Turkey has yet to spell out its legal moves and their bases in detail, Turkey’s implementing a total ban on warships transiting the straits is incompatible with the convention.
Under Article 19, Turkey cannot close the straits to all warships. The right of nonbelligerent warships to transit is enshrined in Article 19(1). Even if Turkey never introduces formal rules closing the straits to warships, its “warning” to all states could be seen as an attempt to impede and deter traffic, and thus a de facto closure. At the moment, the only littoral states party to the war are Ukraine and Russia. If Turkey wants to implement Article 19 faithfully, it must clarify its position and narrowly apply the closure only to belligerent warships, subject to the two exceptions of Article 19(4) mentioned above.
Turkey could close the straits to all warships if it invoked Article 21. However, to do this in conformity with the convention, Turkey would have to take several actions.
Turkey would first have to consider itself “threatened with imminent danger of war.” International law has a well-established definition of “imminent threat of use of force.” Turkey has made no indication that it considers itself subject to an imminent threat of war, nor has it indicated who threatens war on Turkey. Even if Turkey had claimed to be the target of an imminent threat of war, Article 21 requires Turkey to notify other High Contracting Parties of its decision to invoke Article 21. Invoking Article 21 would also likely allow any convention party to refer the matter to the U.N. General Assembly for review, potentially leading to a block on Turkish action under Article 21.
On either basis, Turkey’s staying its latest course would create a precedent that allows Turkey to close at will the straits entirely to foreign warships. If rooted in Article 19, closing the straits to all warships would create a precedent of Turkey closing the straits entirely during any interstate war. This could, as in the current case, impede collective defense and deterrence efforts. In contrast, if Turkey justifies its current direction with Article 21 with no reference, let alone credible reference, to an imminent threat of war, in addition to no apparent notification, the door could open to further abuses of Article 21. Either way, states reliant on transiting the straits, including both Russia and NATO states, might see the Montreux regime as actively harming their security interests.
Invoking and properly implementing Article 19 will likely have little impact on the military balance in Ukraine, but it could create some headaches for military planners in Russia if the conflict persists. And it could also bolster Turkish maritime security.
Implementing Article 19 and Article 19(4) would normally permit Russia to send at most five additional warships (including auxiliary vessels) into the Black Sea and at least 16 warships out of the Black Sea. According to Russian Fleet Analysis, a flotilla of 14 Russian warships, many drawn from the Northern and Pacific fleets, is currently in the Eastern Mediterranean. Among these ships are five assigned to the Black Sea Fleet that would also have a right to reenter the Black Sea. All other Russian warships last known to be in the Eastern Mediterranean have no right to enter the Black Sea under Article 19. Among Russian warships currently in the Black Sea, the 16 warships from other fleets that entered the Black Sea before the invasion began would have a right to leave the Black Sea. Any Russian warships assigned to the Black Sea Fleet and in the Black Sea currently, however, would have no right to exit.
Turkey appears to be taking a traditional approach to Article 19(4) with respect to these ships. Turkey reportedly received requests for transit for four Russian warships to enter the Black Sea, but on March 1, Turkey announced that it had denied Russia’s request, noting that three of the ships were not based at Sevastopol and therefore not covered by the Article 19(4) exception. Russia reportedly acquiesced to Turkey’s refusal.
Moving the five vessels eligible for entry into the Black Sea under Article 19(4) would lock the entire Black Sea Fleet into the Black Sea for the remainder of the conflict. Any maritime presence in the Eastern Mediterranean would have to come from other, more geographically distant fleets. Moreover, if the war drags on, Russian warships in the Black Sea would not be able to return to shipyards elsewhere for refits and repairs. And Russia would not be able to reinforce its presence off Ukraine if it so desired as the war progresses.
Turkey’s current course aims to deny access to the Black Sea for NATO warships, but under a lawful implementation of Article 19 these nations should retain the freedom to navigate through the straits as long as NATO remains nonbelligerent. This departure from the convention could have operational consequences for NATO’s ability to protect Black Sea NATO states, including Turkey, if Russia-NATO ties deteriorate further.
Ankara’s claims that it is implementing the convention while in fact departing dramatically from it could be motivated by three diplomatic concerns.
First, Turkey could see its unlawful approach to warship navigation restrictions as tamping down possible NATO-Russia escalation. On Feb. 28, Turkish President Recep Tayyip Erdogan noted that Turkey would use the convention to minimize escalation risks. If NATO states increased their temporary maritime presence in the Black Sea, Russia might misread this as a prelude to a NATO military operation. However, the escalation risk of any possible NATO warship visits to Romania seems minor compared to NATO’s muscular posture in Poland and other eastern NATO members.
Second, Turkey might also be disposed toward an unlawful, blanket ban on warship transits if Ankara thinks that discriminating between belligerents and nonbelligerents, as Article 19 requires, would provoke Russia to denounce the Montreux Convention. However, Russia would be extremely reckless to denounce Montreux because Russia is arguably the prime beneficiary of the Montreux regime. Without Montreux, the U.N. Convention on the Law of the Sea’s rules on transit passage would apply to the straits. That would mean that all warships would have unrestricted access to the Black Sea and NATO could significantly increase its maritime presence in the Black Sea.
Finally, Turkey might also believe that taking a nondiscriminatory approach to closing the straits to warships would minimize the action’s impact on Turkey-Russia relations. Russia might see Turkey’s closing the straits only to Russian warships as “picking a side.” This is perhaps the best logic for Turkey’s unlawful approach to warship transits.
Turkey Should Clarify Its Position
Cavusoglu’s Feb. 28 comments that suggest the straits are closed to all warships cannot be justified under Article 19 of the Montreux Convention, and there is no indication that Turkey has invoked Article 21. While this course of action might make some diplomatic sense for Turkey, it could risk the long-term security of the convention when better options exist.
Publicly, Turkey should quickly and clearly invoke Article 19 as the convention requires. And under Article 19, Russian and Ukrainian warships must generally be denied transit, unless returning to their bases. Turkey should clearly acknowledge the right of all other warships to transit the straits. This is the best way to safeguard the convention’s long-term legal integrity. Ankara’s diplomatic concerns can best be served by diplomacy behind the scenes: namely, an understanding that NATO states will not try to transit the straits and communicating this to Moscow.
The United States and Germany thanked Turkey for its management of the straits. However, absent clarifying statements from Turkey, the United States and other countries concerned about navigational rights should press Turkey to explain the basis for its restrictions. Although minor compared to the conflagration in Ukraine, letting Turkey fudge the convention now could undermine its stability, and that of a law-based international system, in the years to come.