The 1936 Montreux Convention Explained: Türkiye, the Straits and Black Sea Security

Why It Matters

Türkiye cannot simply close the Bosphorus and Dardanelles whenever it wishes. The 1936 Montreux Convention guarantees free civilian transit in peacetime, caps the size and number of foreign warships, and gives Ankara broad discretion over warship passage only in wartime situations. Since February 2022 it has been the main legal tool limiting naval escalation in the Black Sea, which is why it is now cited far beyond maritime law circles.

The short answer to the question readers ask most: Montreux allows free merchant passage in peace, regulated passage for warships, and, under Article 19, a ban on belligerents' warships when Türkiye is neutral in a war. Details follow below.

Background

From Lausanne to Montreux

The Turkish Straits consist of the Bosphorus (roughly 31 km / 19 miles long, about 700 m / 2,300 ft wide at its narrowest), the Sea of Marmara, and the Dardanelles (roughly 61 km / 38 miles long). Together they are the only maritime exit from the Black Sea to the Mediterranean, and Istanbul sits astride the Bosphorus.

The regime of the Straits was long a European power-politics question. The 1923 Straits Convention signed alongside the Treaty of Lausanne demilitarized the zone and placed it under an international commission. Türkiye argued in the 1930s that this arrangement no longer fit Europe's security conditions and requested a new conference.

The Montreux conference ended with signature on 20 July 1936, and the Convention entered into force on 9 November 1936. The original parties were Türkiye, the United Kingdom, France, the Soviet Union, Bulgaria, Greece, Japan, Romania and Yugoslavia; Italy acceded in 1938. The Convention restored full Turkish sovereignty over the Straits zone, including the right to remilitarize it, while preserving the principle of freedom of transit. Article 28 allows denunciation after twenty years on two years' notice; no party has ever done so.

Merchant ships

In peacetime, merchant vessels of any flag and any cargo enjoy freedom of transit and navigation by day and night, subject to sanitary controls. Fees are limited to the services and tariffs listed in the Convention's annex. The Convention's rules shift in wartime:

• Türkiye not at war, but others are: merchant vessels of any flag keep freedom of transit.
• Türkiye at war: merchant ships not belonging to a country at war with Türkiye may pass, provided they do not assist the enemy.
• Türkiye threatened with imminent danger of war: transit continues, but ships must enter by day and follow the route Turkish authorities indicate.

Separately, Türkiye applies national Maritime Traffic Regulations for the Straits (introduced in 1994 and revised in 1998) for navigational safety and environmental protection in one of the world's busiest and most congested waterways.

Warships: classes, notice and tonnage

Montreux distinguishes between Black Sea (littoral) powers and non-Black Sea powers, and between ship classes. The main peacetime rules are:

These limits mean that large non-littoral fleets, including aircraft carriers, cannot station themselves in the Black Sea, while the littoral states retain their naval dominance there. During the Cold War, Türkiye handled the Soviet Union's Kiev-class ships by classifying them as anti-submarine cruisers rather than aircraft carriers.

Wartime: Articles 19, 20 and 21

• Article 19 (Türkiye not belligerent): warships have the same freedom of transit as in peacetime, but warships of belligerent states may not pass. The Convention provides narrow exceptions: assistance to a victim of aggression under a mutual-assistance treaty binding Türkiye within the League of Nations Covenant framework, and ships returning to their home bases.
• Article 20 (Türkiye belligerent): passage of warships is left entirely to the discretion of the Turkish government.
• Article 21 (Türkiye threatened with imminent danger of war): Türkiye may apply Article 20-style discretion.

Can Türkiye close the Bosphorus and Dardanelles?

In peacetime, no. Türkiye may regulate safety and environmental matters and must enforce the warship limits, but it may not stop lawful merchant traffic. When Türkiye is a non-belligerent in someone else's war, it must close the Straits to belligerents' warships (Article 19), not to all shipping. Only if Türkiye is at war or imminently threatened does it gain the broad discretion of Articles 20 and 21. The Convention does not define 'war', so the determination rests in practice with Ankara and has been debated by scholars.

Key Players & Positions

Russia. As the Soviet Union's successor and the largest Black Sea navy, Russia has traditionally valued Montreux because it limits non-littoral fleets in the Black Sea. Russian officials have supported maintaining the regime and have opposed any revision. Russia has also pointed to the home-base exception in Article 19 as a legitimate basis for some of its ships' movements.

Ukraine. Ukraine is not an original party to the Convention but is a Black Sea coastal state directly affected by it. On 26 February 2022, Ukraine's foreign minister publicly called on Türkiye to close the Straits to Russian warships. Kyiv has since argued that the Convention should be applied as strictly as possible against Russian naval movements.

NATO states. Türkiye, Bulgaria and Romania are NATO's Black Sea littoral members; the other allies are non-littoral and therefore subject to Montreux's tonnage and duration limits. Allied governments have generally accepted that the Convention governs their naval access and have not called for its revision. On 11 January 2024, Türkiye, Romania and Bulgaria agreed in Istanbul to set up a Black Sea mine countermeasures task group to clear drifting mines, an effort coordinated within Montreux limits.

Legal scholars. Commentators broadly agree on the Convention's structure but dispute its limits. Common points of debate include:

• Whether the Convention's undefined notion of 'war' lets Türkiye determine unilaterally when Article 19 applies.
• How broadly the home-base exception should be read, and whether it can be used to move ships that were deployed elsewhere during hostilities.
• Whether Article 19's mutual-assistance exception, tied to the League of Nations Covenant, has any practical meaning after the League's dissolution.
• How the Convention relates to obligations under the UN Charter.

These are interpretive debates; there has been no binding international court ruling on them.

Source Comparison

Readers can verify the rules directly: the Convention text is published in the League of Nations Treaty Series, and the Turkish Foreign Ministry publishes the English text and its notes on the Straits. Gazetteer of Nations keeps statements of fact and statements of position separate, as described in Editorial Principles: How Gazetteer of Nations Separates Fact from Position.

Key Terms & Related Guides

• Turkish Straits: the Bosphorus, the Sea of Marmara and the Dardanelles, defined collectively in the Convention.
• Black Sea powers: states with a Black Sea coastline (Türkiye, Russia, Ukraine, Romania, Bulgaria, Georgia). Only some are original Convention parties, but the regime governs access for all.
• Article 19: the clause barring warships of belligerent states from the Straits when Türkiye is not a belligerent.
• Tonnage limits: the 15,000-tonne transit cap and the 30,000-tonne (up to 45,000-tonne) aggregate cap for non-littoral navies in the Black Sea.
• Home-base exception: the allowance for vessels to return to their home ports.

Related guides: Gazetteer of Nations home, Article 51 of the UN Charter and Türkiye's Cross-Border Operations in Syria and Iraq, and Türkiye's Global Diplomacy.

Perspective Note

Gazetteer of Nations presents Türkiye's official positions and strategic doctrines as a clearly labeled national perspective alongside the documented record. Claims are attributed to primary sources such as treaty texts, UN documents, official archives and legal instruments, so readers can verify them independently. Author: Gazetteer of Nations Editorial Team.

Cross-Publication Dossiers & Sister Desk Perspectives

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

• The Nations Dialogue: The 1936 Montreux Convention: How Türkiye Governs the Turkish Straits and Black Sea Security — Montreux Convention Turkish Straits Black Sea Article 19 explained: how the 1936 treaty governs merchant and warship passage and how Türkiye applied it.
• Bloc Politics: The 1936 Montreux Convention Explained: How Türkiye Governs the Turkish Straits and Black Sea Access — How the 1936 Montreux Convention governs the Bosphorus and Dardanelles: merchant freedom, warship limits, Article 19 and Türkiye's 2022 application.
• Horizon of Nations: The 1936 Montreux Convention: How Türkiye Manages the Turkish Straits in the Black Sea War — How the Montreux Convention governs the Turkish Straits: commercial passage, warship limits, Article 19 in the Black Sea war, and the views of all sides.