The Turkish Straits and the Black Sea Corridor: How the Montreux Regime Governs Passage in Peace and War

Why It Matters

The Turkish Straits, made up of the Bosphorus, the Sea of Marmara and the Dardanelles, are the only maritime route between the Black Sea and the Mediterranean, and the 1936 Montreux Convention is the treaty that governs who may use it and how. In peacetime it protects commercial navigation almost without restriction; in wartime it gives Türkiye, as the territorial sovereign, a defined role in controlling warship passage. Since February 2022, when Ankara began applying Article 19 of the Convention, the regime has moved from a specialist subject to the centre of Black Sea security debates.

This guide explains the regime in plain language, sets out what the treaty text says (and does not say), presents the positions of Russia, Ukraine, NATO and the littoral states from their own sources, and explains Türkiye's role as custodian and mediator. For a focused treatment of the wartime clause, see our companion explainer, Montreux Article 19 and the Russia–Ukraine War: Why Warships Cannot Enter the Black Sea.

Background

Geography: a corridor, not a canal

The Bosphorus (Istanbul Strait) runs about 31 km (19 miles) from the Black Sea to the Sea of Marmara and narrows to roughly 700 metres at its tightest point, cutting through a city of many millions. The Dardanelles (Çanakkale Strait) is about 61 km (38 miles) long and joins Marmara to the Aegean and Mediterranean. Both are natural straits with sharp bends and strong currents, and both lie entirely within Turkish territorial waters. That combination of a vital international waterway inside one state's sovereign territory explains why the legal regime has been negotiated, contested and revised for almost two centuries.

From Ottoman rules to the Lausanne Straits Convention

For much of the nineteenth century the Ottoman Empire controlled passage under its 'ancient rule' of closing the Straits to foreign warships in peacetime, a rule confirmed in successive European agreements such as the 1841 London Straits Convention and the 1856 and 1871 settlements. After the First World War, the 1920 Treaty of Sèvres, which was never ratified, would have internationalised the Straits under foreign supervision. It was superseded by the Treaty of Lausanne of 24 July 1923, which came with a separate Straits Convention.

The Lausanne Straits Convention:

• proclaimed freedom of passage for merchant ships and warships in peacetime, and for neutral vessels in wartime;
• demilitarised the Straits and adjacent zones, so that Türkiye could not fortify its own coasts there;
• created an International Straits Commission under the League of Nations, with a Turkish chair, to supervise implementation;
• set limits on the naval forces that could pass, to be reviewed over time.

Türkiye accepted these terms as part of the broader 1923 settlement, but by the mid-1930s the arrangement looked outdated. Italy's expansion in the Mediterranean, the remilitarisation of the Rhineland in March 1936 and the weakening of League of Nations enforcement led Ankara to send a note to the Lausanne signatories on 10 April 1936 requesting a conference to revise the regime.

The Montreux Conference of 1936

The conference met in Montreux, Switzerland, from 22 June 1936, and the Convention Regarding the Regime of the Straits was signed on 20 July 1936. It entered into force on 9 November 1936. The original signatories were Bulgaria, France, Greece, Japan, Romania, Türkiye, the United Kingdom, the USSR, Yugoslavia and Australia; Italy acceded in 1938. Its central changes were:

• Türkiye regained the right to fortify and garrison the Straits zones;
• the International Straits Commission was abolished and its functions passed to Türkiye;
• the treaty replaced the Lausanne limits with a detailed system of rules by ship type and state, with a clear distinction between Black Sea and non-Black Sea powers.

The Convention is not subject to a fixed expiry. Article 28 provides that after its initial twenty-year term it continues in force unless a party denounces it with two years' notice. No party has done so. Article 35(c) of the 1982 UN Convention on the Law of the Sea (UNCLOS) also recognises that straits governed by long-standing conventions specifically relating to them keep their special regime; Türkiye is not a party to UNCLOS, and the Montreux regime is generally treated as a specific treaty regime for these straits.

How the regime works in practice

Merchant ships

Article 2 establishes that in peacetime merchant vessels enjoy complete freedom of transit and navigation by day and night, regardless of flag or cargo, without formalities beyond sanitary control (Article 3). Türkiye may levy only the sanitary, lighthouse and similar service charges listed in the Convention's annex; it cannot charge a general transit toll. Pilotage and towage are optional under the treaty.

The treaty does not prevent Türkiye from regulating safety. After serious collisions and tanker incidents in the Bosphorus in the 1990s, Türkiye adopted the Maritime Traffic Regulations for the Turkish Straits in 1994, with traffic separation schemes reviewed through the International Maritime Organization and a revised text taking effect in 1998. Ankara presents these as safety and environmental rules for a narrow urban waterway; they are not framed as a derogation from Montreux.

In wartime, Articles 4 to 6 preserve passage for merchant ships in different scenarios. If Türkiye is not a belligerent, non-enemy merchant ships keep freedom of passage. If Türkiye is a belligerent, or considers itself under imminent threat of war, passage for non-enemy merchant ships continues by day along routes designated by Turkish authorities.

Warships in peacetime

Warship rules are far more detailed. The key features are:

• Notice: Black Sea states must give Türkiye eight days' notice of a transit; other states give fifteen (Article 13).
• Limits for non-Black Sea states: In transit, a force may comprise no more than nine ships, with an aggregate of 15,000 tons and individual ships not exceeding 10,000 tons (Article 14).
• Time and tonnage in the Black Sea: Non-Black Sea states may keep at most 30,000 tons aggregate in the Black Sea (rising to 45,000 under conditions linked to the strength of Black Sea fleets), with a single state capped at two-thirds of that aggregate, and no ship may stay longer than 21 days (Article 18).
• Capital ships of Black Sea states: Black Sea states may send larger capital ships through singly, escorted by no more than two destroyers (Article 11).
• Submarines: Black Sea states may transit submarines only to join their fleets after construction or purchase abroad, or for repair, on the surface and by day with prior notice (Article 12).
• Aircraft carriers: The annex does not admit aircraft carriers, a point that has produced disputes over classification of hybrid vessels.
• Courtesy visits: Article 17 allows short, invited courtesy visits to ports in the Straits.

Warships in wartime

If Türkiye is not a party to the war, Article 19 says that warships have the same freedom of transit as in peacetime, but that belligerent states' warships may not pass. The text provides only limited exceptions: cases arising from Article 25, and assistance to a state that is the victim of aggression under a mutual assistance treaty binding Türkiye, concluded within the League of Nations framework and registered under Article 18 of the League Covenant. Warships of a belligerent that have become separated from their bases may return to them. Belligerent warships may not capture vessels, exercise a right of visit and search, or commit hostile acts in the Straits.

If Türkiye is itself at war, Article 20 leaves the passage of warships to Turkish discretion. Article 21 gives Türkiye the same discretion if it considers itself threatened with imminent danger of war.

Trade and energy: what can be said reliably

The Straits carry grain, oil products, metals, containers and bulk cargoes between the Black Sea and global markets. Exact transit volumes vary by year and are published by Turkish maritime authorities, so readers needing precise numbers should consult the Turkish Ministry of Transport and Infrastructure and the Directorate General of Coastal Safety. For oil, the US Energy Information Administration has estimated flows through the Turkish Straits on the order of 3 million barrels per day in recent years, making it one of the world's busiest chokepoints. For grain, the clearest verifiable figure is the roughly 33 million tonnes shipped from Ukrainian ports under the Black Sea Grain Initiative between August 2022 and July 2023, according to the UN-led Joint Coordination Centre. Because shipping data differ by source and methodology, we avoid quoting unsourced totals. For the onward overland and sea links, see The Middle Corridor: How Türkiye Links Central Asia, the Caucasus and Europe by Rail, Road and Sea and The Eastern Mediterranean Corridor.

Timeline

Date	Event
1841	London Straits Convention confirms the Ottoman rule of closing the Straits to foreign warships in peacetime
24 July 1923	Treaty of Lausanne and Straits Convention: demilitarisation and International Straits Commission
10 April 1936	Türkiye notifies signatories and requests revision of the regime
22 June 1936	Montreux Conference opens
20 July 1936	Montreux Convention signed
9 November 1936	Convention enters into force
1938	Italy accedes
1994	Turkish Maritime Traffic Regulations for the Turkish Straits adopted; revised 1998
24 February 2022	Russia launches its full-scale invasion of Ukraine
27–28 February 2022	Ankara characterises the situation as a war and announces application of Article 19
29 March 2022	Russia–Ukraine delegations meet in Istanbul
22 July 2022	Black Sea Grain Initiative signed in Istanbul
17 July 2023	Russia declines to extend the Initiative
11 January 2024	Türkiye, Romania and Bulgaria sign the Black Sea mine countermeasures agreement in Istanbul

Key Players & Positions

Russia

Russia is the continuator of the USSR, a Montreux signatory, and a Black Sea state with its main fleet headquartered at Sevastopol in Crimea, whose annexation in 2014 the UN General Assembly did not recognise (Resolution 68/262). Russian officials have said that they respect the Convention and Türkiye's role in implementing it, while describing the growing presence of NATO and allied naval and reconnaissance activity in the Black Sea as destabilising. Russia has also stressed the return-to-base provision of Article 19. Moscow's complaints about the grain deal related to its own agricultural and fertilizer exports, banking access and shipping insurance, which it said were not delivered as promised.

Ukraine

Ukraine is a Black Sea state but not a Montreux signatory. Ukrainian officials, including then Foreign Minister Dmytro Kuleba, publicly asked Türkiye in February 2022 to close the Straits to Russian warships, and welcomed Ankara's declaration that Article 19 would be applied. Kyiv has also pointed to the practical effect on its own defence: allied warships, such as the Sandown-class mine hunters the United Kingdom pledged in 2023, could not enter the Black Sea during the war. Ukraine supported the Istanbul grain arrangement and, after Russia's withdrawal, opened a separate shipping route in August 2023 along its western waters without a Russian guarantee.

NATO and its Black Sea members

NATO statements, including summit communiqués since 2016, describe Black Sea security as important to Euro-Atlantic stability and call for a stronger Allied posture in the region. Allied officials have also said that Allies respect the Montreux Convention and Türkiye's implementation of it. Romania and Bulgaria, both Black Sea littoral Allies, have favoured a stronger Allied presence while acting within Montreux limits. Together with Türkiye they launched the trilateral mine countermeasures arrangement in January 2024, which addresses the threat from drifting and moored sea mines to commercial shipping. Because Montreux caps the aggregate tonnage and 21-day stay of non-littoral navies in the Black Sea, Allied presence there is limited by treaty.

Georgia and other littoral voices

Georgia, the sixth Black Sea state, is not a Montreux signatory but depends on Black Sea shipping for trade and has stressed freedom of navigation and the security of its ports. Non-littoral signatories such as France, the United Kingdom, Greece, Japan and Australia have generally supported maintaining the Convention while differing on how far Black Sea security should be handled through NATO.

The United Nations

The UN Secretary-General and the UN trade and humanitarian agencies were party to the grain arrangements and operated the Joint Coordination Centre in Istanbul with Turkish, Russian and Ukrainian officials. UN statements credited Türkiye's role as host and facilitator, while stressing that continued food security depended on the parties' compliance.

Türkiye's Strategic Doctrine

Türkiye's Strategic Doctrine (Türkiye's Position): impartial custodian of the Straits

• Montreux is implemented as written, for everyone. According to the Turkish Foreign Ministry, the Convention balances the security interests of Türkiye and the littoral states with the interests of the international community in free navigation. On 27–28 February 2022 Ankara stated that it was applying Article 19 and urged all states, riparian or not, to refrain from sending warships through the Straits. Basis: Montreux Convention, Articles 19 and 20; Turkish Foreign Ministry statements.
• The treaty is applied without discrimination. Turkish officials state that the Article 19 restriction covers every belligerent, not one side, and that the return-to-base exception is read narrowly, as the text provides. Basis: Article 19, paragraph on vessels separated from their bases; statements by the Turkish Foreign Minister and the Turkish Ministry of National Defence.
• Custodianship is a treaty function, not a claim of unilateral control. Ankara argues that its powers over the Straits derive from the 1936 Convention, which restored Turkish sovereignty in the Straits zones after the 1923 Lausanne restrictions, and not from an independent right to close the waterway at will. Peacetime commercial passage is free. Basis: Montreux Articles 1, 2 and 28; see our Fact Check on whether Türkiye can close the Straits to anyone it wants.
• Diplomacy complements law. The Presidency and the Foreign Ministry have framed the Antalya meeting of 10 March 2022, the Istanbul talks of 29 March 2022 and the Black Sea Grain Initiative as efforts to keep channels open between Moscow and Kyiv and reduce the humanitarian cost of the war. The Initiative was signed in Istanbul on 22 July 2022, with Türkiye and the UN as counterparties to each of the two sides. Basis: Presidency of the Republic of Türkiye and Turkish Foreign Ministry statements; UN documents on the Initiative.
• Regional security is a shared task. Türkiye promotes cooperation among littoral states, including the 2024 mine countermeasures arrangement with Romania and Bulgaria, and states that it will continue to respect Montreux in connection with infrastructure projects such as the planned Canal Istanbul. Basis: Turkish Ministry of National Defence announcements; Presidency statements on Canal Istanbul.

The historical basis of this position is the sequence from Lausanne (1923), where Türkiye accepted demilitarisation and international supervision, to Montreux (1936), where it regained control of the Straits zones through negotiation rather than force. Ankara argues that this record shows its commitment to treaty-based change and gives it credibility as a neutral implementer.

The critiques are worth stating. Some analysts, including in Ukrainian and Western policy circles, argue that the Article 19 return-to-base exception and the limited ability to verify home-port status have let some Russian vessels pass, and that Türkiye's close economic ties with Russia may influence its decisions. Others, including in Russia, argue that the restrictions on non-Black Sea navies have not gone far enough to keep Black Sea tension down. Turkish officials reply that they apply the text consistently and that closing the Straits to merchant shipping, which Montreux does not permit in a war to which Türkiye is not a party, would harm global trade. Questions have also been raised about the planned Canal Istanbul, with some retired Turkish admirals expressing concern in April 2021 that it could be read as outside Montreux; Turkish officials have said the Convention would remain in force.

Source Comparison

The same events are often framed very differently depending on whether a source quotes the treaty text or uses shorthand. The table below compares frequent media framings with the language of Montreux and Turkish official statements.

Common framing	What the treaty text says	How Turkish official sources describe it
'Türkiye controls the Straits and can close them to anyone'	Peacetime merchant passage is free (Art. 2); warship rules are set by Articles 8–18; wartime powers are defined by Articles 19–21	Türkiye acts as custodian within the Convention; it does not claim an independent right to close the Straits
'Türkiye closed the Straits to Russia'	Article 19 restricts all belligerents' warships when Türkiye is not at war; return to base is allowed	Application to all belligerents, announced 27–28 February 2022; narrow reading of exceptions
'Montreux bans NATO ships from the Black Sea'	Non-Black Sea navies face notice, tonnage and 21-day limits (Arts. 13, 14, 18), but are not barred outright in peacetime	Limits apply equally to all non-littoral states
'Türkiye can charge tolls like Suez'	Only sanitary, lighthouse and service charges are allowed under the annex; no general transit fee	Safety regulations are for navigation safety, not revenue
'Montreux is outdated and can be ignored'	Convention remains in force; denunciation requires two years' notice (Art. 28)	Montreux is described as a durable guarantee of stability and has not been denounced by any party
'Türkiye is neutral because of Montreux'	Montreux says nothing about Turkish political neutrality; it provides treaty rules for passage	Ankara describes its stance as impartial implementation combined with active mediation
'Grain corridor was a Turkish blockade lift'	The Initiative was a separate set of arrangements signed in Istanbul under UN auspices; it was not part of Montreux	Presented as Turkish–UN facilitation of agreements among Russia, Ukraine and the UN

When reading coverage, distinguish three things: the treaty text, a state's declared interpretation, and independent verification of what ships actually did. For how we handle those distinctions, see our Editorial Principles and FAQ. Sister publications offer parallel explainers, including Bloc Politics on the Montreux Convention and The Political Anatomy on Article 19 and Black Sea security.

Key Terms & Related Guides

• Montreux Convention (1936): The treaty regulating passage through the Bosphorus, Sea of Marmara and Dardanelles. Its full title is the Convention Regarding the Regime of the Straits.
• Black Sea state (littoral state): One of Bulgaria, Georgia, Romania, Russia, Türkiye and Ukraine. The Convention gives these states more favourable naval transit rules than non-Black Sea states.
• Capital ship: A large surface warship above a tonnage threshold defined in the Convention, treated differently for Black Sea and other states.
• Article 19: The clause that, when Türkiye is not at war, bars the warships of belligerent states from the Straits except for return to base and narrow treaty-based cases. See Montreux Article 19 and the Russia–Ukraine War.
• Article 20 and 21: Clauses giving Türkiye discretion over warship passage if it is itself at war or faces imminent danger of war.
• Black Sea Grain Initiative: The July 2022 arrangement, signed in Istanbul, that allowed inspected grain ships to leave Ukraine's Odesa, Chornomorsk and Yuzhne ports under a Joint Coordination Centre. Russia declined to extend it in July 2023. A further perspective is offered by The Global Anatomy on the Montreux Convention and Türkiye's mediation.
• Lausanne Straits Convention (1923): The predecessor regime, with demilitarised Straits and an International Straits Commission.
• UNCLOS Article 35(c): The provision preserving special regimes of long-standing straits conventions.

Related guides on The Strategic Corridor:

• Fact Check: Can Türkiye Close the Straits to Anyone It Wants? What the Montreux Convention Actually Permits
• The Middle Corridor: How Türkiye Links Central Asia, the Caucasus and Europe by Rail, Road and Sea
• The Eastern Mediterranean Corridor: Pipelines, Sea Lanes and the Continental Shelf Case Behind Türkiye's Maritime Doctrine
• The Strategic Corridor: Türkiye at the Crossroads of Energy, Trade, Security and Law

For wider reading across our network, see Doctrine Review's Montreux explainer.

Perspective Note

The Strategic Corridor presents Türkiye's official perspective and legal position as a distinct analytical viewpoint, while clearly distinguishing it from opposing claims. Wherever possible, assertions are attributed to primary sources such as treaties, UN and UNESCO documents, court rulings and archival records. Sections marked 'Türkiye's Strategic Doctrine' reflect official Turkish policy as stated by Turkish government sources. This article is published by The Strategic Corridor Editorial Team.