On Sunday, Turkish leaders labeled Russian’s invasion of Ukraine a war, a rhetorical shift that sets the stage for Turkey limiting warships transiting the Turkish Straits and entering the Black Sea. Speaking on CNN Turk, Turkish Foreign Minister Mevlüt Çavuşoğlu stated that “the situation in Ukraine has transformed into a war” and Turkey “will implement all articles of Montreux transparently.” Çavuşoğlu was referencing the 1936 Montreux Convention, an international agreement that governs the transit of all vessels and airplanes through the Turkish Straits, a strategic chokepoint that links the Black Sea with the Mediterranean Sea. As the Russia-Ukraine conflict rages, the Montreux Convention has taken on increased importance as a potential regulator of warship traffic into the conflict zone. If Turkey formally invokes Montreux’s wartime provisions, Russian warships will generally be prohibited from entering the Black Sea. This would play a small but substantive role in de-escalating Russia-Ukraine tensions.

What is the Montreux Convention?

The 1936 Convention Regarding the Regime of the Straits (commonly referred to as the Montreux Convention, after the city in Switzerland where it was negotiated) is a 1936 international agreement that governs the transit of the Turkish Straits for merchant vessels, vessels of war, and aircraft. Negotiated in the shadow of an expansionist Nazi Germany, the Convention includes 29 Articles and three technical annexes that address which warships may enter the Black Sea.  It has played an important role in demilitarizing the Black Sea for the past 85+ years. It does so by limiting the size of vessels that may enter the Black Sea, imposing notification requirements on warships transiting the Turkish Straits, and restricting how long non-Black Sea powers can deploy their warships in the Black Sea. Since its inception, Montreux has played an important role in enforcing a rules-based international order in the Black Sea and Turkish Straits.

The U.N. Convention on the Law of the Sea (UNCLOS), the “Constitution of the Oceans”, governs transit passage through international straits around the world. Article 35 clarifies that UNCLOS does not apply to “long-standing international conventions in force.” The upshot: Montreux Convention’s restrictive provisions, and not UNCLOS, govern the Turkish Straits, which enjoy a truly unique legal status in international transit governance.

Because Montreux was negotiated over 85 years ago and naval warships and technology have changed dramatically since that time, it can sometimes be difficult to apply the Convention’s highly technical transit limitations to modern warships. Montreux generally limits the passage of capital ships greater than 15,000 tons, but this can be subject to some interpretation and debate. For example, during my time on the U.S. Navy’s Sixth Fleet staff in 2008, where I served as an international law and ethics attorney, the Russia-Georgia conflict raged for several weeks. The USS Mount Whitney transited the Straits and entered the Black Sea despite questions about its tonnage and whether it fell under a Montreux exception. While Russia protested the Mount Whitney’s transit as a violation of Montreux and an unnecessary provocation, Turkey has allowed the Mount Whitney to transit the Turkish Straits since that time. U.S. destroyers routinely transit the Straits. The USS Porter, a guided missile destroyer, conducted exercises and operations in November 2021 with the Bulgarian, Romanian, Turkish, and Ukrainian navies.

During peacetime, civilian merchant vessels enjoy complete freedom of transit and navigation through the Straits, “under any flag any with any kind of cargo.” The Montreux Convention is unique in the law of the sea in that it explicitly discriminates against nations outside the Black Sea. Instead, Montreux favors six “Black Sea Powers” (Russia, Ukraine, Turkey, Romania, Georgia, and Bulgaria) over “non-Black Sea Powers” (everyone else). Non-Black Sea Powers cannot remain in the Black Sea for longer than 21 days. The Convention also restricts access to the Black Sea for belligerents during war and allows Turkey to restrict access when it considers itself to be threatened with imminent danger of war. Following Russia’s annexation of Crimea, its naval power and presence in the Black Sea has increased – which is permissible under the privileges it enjoys as a Black Sea Power, and which has been a source of growing NATO concern.

What is the Significance of Montreux’s Wartime Provisions?

Of the Black Sea Powers, Turkey is, by far, the most important. Montreux empowers Turkey with the legal authority to control the passage of warships through the Straits during war. Were Turkey to restrict passage now, that would hurt Russia but not affect Ukraine, which lacks a large naval presence and is operating in a defensive posture. In fact, Ukraine has already asked Turkey to intervene by closing the Straits to Russian warships. The Black Sea has been the site of fighting between Russia and Ukrainian forces, and several Russian naval assets are already in the Black Sea.

Two Montreux provisions are of critical importance for the unfolding Russia-Ukraine conflict. First, Article 19 states that during times of war – when Turkey is not a belligerent – warships enjoy freedom of navigation and transit through the Straits. But vessels of war belonging to belligerent parties are prohibited from transiting the Straits unless they are returning to base or rendering assistance. This appears to be the situation at hand in Ukraine, as we are witnessing two sovereign nations engaged in an international armed conflict. Turkey is not actively fighting and remains a non-belligerent. The Turkish Foreign Minister pointed to this provision yesterday, stating “Article 19 of the Montreux Convention is clear. This is war.” Invoking Article 19 would restrict Russian warships operating in the Mediterranean Sea from entering the Black Sea. This is significant: as of this writing, there are 16 Russian warships – the bulk of the entire Russian Navy ­– operating off the coast of Syria. They would now be prevented from transiting the Straits and entering the conflict.

Second, Article 21 states that if Turkey “considers herself to be threatened with imminent danger of war” (emphasis added), the Turkish Government may limit the passage of warships through the Turkish Straits. The decision of what warships may enter is then left entirely to the Turkish Government’s discretion. Once again, if Turkey so elected, this could prevent Russian ships outside the conflict from transiting the Turkish Straits and entering the Black Sea. Significantly, Article 21 empowers Turkey with enormous discretion to allow warships to pass through the Straits. So Turkey could feasibly allow NATO non-Black Sea vessels to transit and exclude Russian vessels from entering the Black Sea. Still, it is hard to see how Turkey could credibly claim that it is threatened with imminent danger of war. Article 19 seems much more likely to be the wartime provision at play.

Invoking either Article would be historically significant. Turkey has not labelled earlier Russian aggression in Georgia, Crimea, or eastern Ukraine a “time of war” and restricted Black Sea access. In fact, Turkey has only invoked Montreux’s wartime provisions once before: in World War II, to prevent German and Italian combatants from entering the Black Sea.

Other Considerations: the U.S. and NATO Maritime Presence and Strategic Concerns Around Escalation 

The U.S. was absent from the Montreux Convention negotiations and has not joined the treaty. Nevertheless, the U.S. abides by the Convention under customary international law. While the USS Harry S. Truman aircraft carrier is operating in the Mediterranean Sea, the Truman’s passage through the Turkish Straits is likely a non-starter. While there is no provision in the body of the Montreux Convention addressing aircraft carriers,  a nuclear aircraft carrier is a capital ship that exceeds Montreux’s tonnage limitations.  But other U.S. and NATO vessels could still feasibly transit the Turkish Straits, provided they give Turkey ample notice under the Convention (15 days for all non-Black Sea Powers). The USS Porter destroyer and USS Mount Whitney (Sixth Fleet flagship) were recently in the Black Sea, although no NATO vessels are operating there now. Vessels of war belonging to non-Black Sea Powers – which includes much of NATO’s maritime force – are prohibited from remaining in the Black Sea for more than 21 days.  Putin may see an increased NATO presence in the Black Sea as an escalatory measure, so it remains unclear whether this is being seriously considered.

If the conflict escalates, the rights of parties other than Ukraine and Russia will also depend on whether other states are adjudged to be co-belligerents. This could cut in one of two ways: if Turkey is not a belligerent but other states join the fight, their access would be compulsorily restricted under Article 19. But if Turkey itself were to join as a belligerent, or if the conflict escalated to the extent that Turkey did feel credibly threatened, Articles 20 and 21 would give Turkey discretion over the passage of warships – including the right to deny return passage to “vessels of war belonging to the State whose attitude has given rise to” Turkey’s belief in imminent danger of war.

Article 23 is the only provision that governs aircraft transiting the Straits. It authorizes Turkey to set up air routes through the Straits. Montreux does not expressly address military aircraft, and it remains unclear how invoking the Article 19 wartime provisions will affect air travel through the Straits.  As Russia is denied access to European airspace, the Turkish Strait air route may well increase in importance.

Finally, it is important to note that Russia, as a Black Sea Power, has benefited quite a bit from the Montreux regime and is almost certain to abide by Turkish authority in the Straits. Disregarding Montreux would immediately escalate tensions between Russia and Turkey and appears to be a non-starter.

It remains to be seen if Turkey’s rhetorical use of war has already led to the legal implementation of the Montreux Convention’s wartime provisions. So far, Turkey has not taken measures to block the Straits, but Çavuşoğlu stated that Turkey always had “implemented the Montreux Agreement to the letter” and would do the same here. And even if Turkey did close the Straits to the arrival of future Russian warships, it could not force Russian vessels already engaging in conflict from leaving the Black Sea. It would also be required to allow those Russian vessels to return home when Russia so chose. Still, invoking the Montreux wartime provision would signal the seriousness with which Turkey, a key NATO ally, views the Russian-Ukraine conflict. It would also be broadly aligned with international efforts to isolate Russia by denying Russian access to airspace and global economic markets.

IMAGE: The Russian Navy’s diesel-electric Kilo class submarine Rostov-on-Don sails through the Bosphorus Strait on the way to the Black Sea past the city Istanbul as Suleymaniye mosque is seen in the backround on February 13, 2022. (Photo by Ozan KOSE / AFP) (Photo by OZAN KOSE/AFP via Getty Images)