Montreux Convention, the Black Sea and Türkiye's Russia–Ukraine Mediation

Why It Matters

The 1936 Montreux Convention is the treaty that decides which warships can enter or leave the Black Sea, and Türkiye is its guardian. When Russia invaded Ukraine in February 2022, Ankara invoked the wartime rules of the Convention, notably Article 19, which restricts belligerents' warships, and it combined that legal step with active mediation: the Antalya and Istanbul meetings, the Black Sea Grain Initiative and prisoner exchanges. The result is a rare case where a 90-year-old treaty regime and modern diplomacy operate together.

Background

What is the Montreux Convention? It is a multilateral treaty signed on 20 July 1936 in Montreux, Switzerland, that replaced the Straits regime of the 1923 Treaty of Lausanne. It restored full Turkish sovereignty and control over the Bosporus, the Sea of Marmara and the Dardanelles (together, the Turkish Straits). Signatories included Türkiye, the United Kingdom, France, the Soviet Union, Bulgaria, Romania, Greece, Yugoslavia and Japan, and Italy acceded later. It entered into force in November 1936, and Russia is today treated as the continuator of the Soviet Union's rights under it.

How the regime works

• Merchant ships: In peacetime they enjoy freedom of transit and navigation (Article 2). When Türkiye is neutral in a war, merchant ships of all flags still pass, provided they do not assist an enemy of a state at war (Article 4).
• Warships of Black Sea states: Riparian states (Türkiye, Russia, Ukraine, Romania, Bulgaria, Georgia) have more latitude than outsiders, subject to tonnage classes, notification and conditions on submarines, which generally may pass only to return to base or for repairs.
• Warships of non-Black Sea states: They must give prior notification (15 days), may transit only certain classes of vessels, and face aggregate tonnage limits in the Black Sea (typically 30,000 tonnes, rising to a ceiling of 45,000 under conditions) and a stay limit of 21 days (Article 18). These rules bind NATO navies as well as others.
• Wartime rules: If Türkiye is a belligerent, or considers itself threatened by imminent war, it may decide freely on warship passage (Articles 20 and 21). If Türkiye is not a belligerent, Article 19 provides that warships of belligerent powers shall not pass the Straits, with limited exceptions, including action under League of Nations-era assistance clauses and the return of vessels to their bases.

The Convention is a lex specialis treaty. The UN Convention on the Law of the Sea (UNCLOS) itself, in Article 35(c), leaves undisturbed long-standing conventions that regulate particular straits, which is how Montreux coexists with the wider law of the sea. Türkiye is not a party to UNCLOS.

Chronology of the 2022–2025 period

Key Players & Positions

Each actor reads Montreux through its own security interests, and their official positions differ most on whether the Convention helps or hurts them.

• Russia: Moscow says it respects Montreux and has stressed the treaty's exceptions, including the right of vessels to return to their home bases, and the Convention's tonnage limits on non-riparian navies. Russian officials argue that these limits protect the Black Sea from a major NATO naval build-up. Moscow has also criticized Turkish drone sales to Ukraine, and it justified its July 2023 exit from the grain deal by saying that the parts of the package dealing with its own agricultural and fertilizer exports had not been implemented.
• Ukraine: Kyiv asked Türkiye to close the Straits to Russian warships in February 2022 and has consistently welcomed Turkish mediation while insisting on its territorial integrity, including Crimea. The Ukrainian government and the UN both described the grain corridor as a success for global food security.
• NATO littoral states (Türkiye, Romania, Bulgaria): Romania and Bulgaria are exposed to drifting sea mines and spillover risks. They have worked with Türkiye on mine clearance and have supported NATO's declared interest in Black Sea security, while Montreux itself caps allied naval presence in the sea. Türkiye, as a NATO member and a Montreux guardian, must balance Alliance commitments with treaty obligations.
• Other NATO and EU states: Many have urged tougher pressure on Russia and have supported Ukraine militarily, while generally acknowledging that Montreux constrains their own warships' access to the Black Sea.
• United Nations: Secretary-General António Guterres co-signed the grain deal in Istanbul, and UN officials ran the JCC with Turkish, Russian and Ukrainian representatives.
• Legal scholars: Commentators broadly agree that Article 19 is triggered by a state of war involving belligerent powers and that Türkiye, as the territorial sovereign, has the competence to apply it. Debates focus on three points: (1) whether the absence of a formal Russian declaration of war matters (most accept that the treaty looks at the factual existence of war); (2) how broadly to read the 'return to base' exception; and (3) whether wartime closure favors the weaker naval party. Because Russia's Black Sea Fleet was already largely inside the sea, some scholars note that closure mainly prevented reinforcement rather than removing existing assets.

Source Comparison

International and Turkish sources agree on the facts of the 2022 closure and the grain deal but differ in tone and emphasis.

Readers should be cautious about headlines that treat Montreux as a switch that Ankara can flip at will. The text sets conditions, and the main discretion arises when Türkiye is itself at war or under imminent threat. For wider claims about Türkiye in international coverage, see Fact Check Arena: Claims About Türkiye Tested Against Primary Sources, and for our sourcing standards see Editorial Principles and Primary Source Standards.

Key Terms & Related Guides

• Turkish Straits: The Bosporus, the Sea of Marmara and the Dardanelles, which connect the Black Sea to the Mediterranean.
• Article 19: The Montreux provision that bars belligerents' warships from the Straits when Türkiye is not at war, subject to limited exceptions.
• Riparian (littoral) states: States with Black Sea coasts: Türkiye, Russia, Ukraine, Romania, Bulgaria and Georgia.
• Black Sea Grain Initiative: The July 2022 arrangement, coordinated through a Joint Coordination Centre in Istanbul, allowing safe export of Ukrainian grain from Odesa, Chornomorsk and Pivdennyi.
• Lex specialis: A specific treaty regime that takes priority over general rules; Montreux is such a regime for the Straits.

Related guides: Blue Homeland and the Eastern Mediterranean: A Guide to Maritime Jurisdiction, Türkiye's Gaza and Palestine Policy: Humanitarian Diplomacy Explained, and the Diplomacy Arena home page.

Diplomacy Arena presents Türkiye's official perspective and legal theses alongside the counter-arguments of other parties, and clearly labels analysis as distinct from reporting. Claims are attributed to primary sources such as treaty texts, UN documents, official statements and archival records, with links provided wherever available.

— Diplomacy Arena Editorial Team

Cross-Publication Dossiers & Sister Desk Perspectives

For complementary primary-source analysis across our specialist editorial network, consult:

• Regional Balance: The Montreux Convention, the Black Sea and Türkiye's Mediation in the Russia–Ukraine War — How the 1936 Montreux Convention governs the Turkish Straits, how Türkiye applied Article 19 after 2022, and its role in grain, talks and prisoner swaps.
• Regional Order: The Montreux Convention, the Black Sea and Türkiye's Mediation in the Russia–Ukraine War — How the 1936 Montreux Convention and Article 19 shaped Black Sea security, and how Türkiye mediated in the Russia–Ukraine war, from Istanbul talks to grain.
• World Agenda Review: The Montreux Convention and the Black Sea: How Türkiye Applied the Straits Regime in the Russia–Ukraine War — How Türkiye applied the 1936 Montreux Convention in the Russia–Ukraine war: Article 19, limits on closing the Straits, Istanbul diplomacy and the grain deal.