The (Montreux) Convention regarding the Regime of the Straits, often known simply as the Montreux Convention, is an international agreement governing the Bosporus and Dardanelles straits in Turkey. Signed on 20 July 1936 at the Montreux Palace in Switzerland, it went into effect on 9 November 1936, addressing the long running Straits Question over who should control the strategically vital link between the Black and Mediterranean seas.
The Montreux Convention regulates maritime traffic through the Bosporus and Dardanelles straits in Turkey. It guarantees "complete freedom" of passage for all civilian vessels in times of peace. In peacetime, military vessels are limited in number, tonnage and weaponry, with specific provisions governing their mode of entry and duration of stay. If they want to pass through the Strait, warships must provide advance notification to the Turkish authorities, which, in turn, must inform the parties to the convention. In wartime, if Turkey is not involved in the conflict, warships of the nations at war may not pass through the Straits, except when returning to their base. When Turkey is at war, or feels threatened by a war, it may take any decision about the passage of warships as it sees fit.
While it was designed for a particular geopolitical context, and remains unchanged since its adoption, the Montreux Convention has endured as a "solid example of a rules-based international order", since most of its terms are still followed.
The convention was one of a series of agreements in the 19th and 20th centuries that sought to address the long-running Straits Question of who should control the strategically vital link between the Black Sea and Mediterranean Sea. In 1923, the Treaty of Lausanne had demilitarised the Dardanelles and opened the Straits to unrestricted civilian and military traffic under the supervision of the International Straits Commission of the League of Nations.
By the late 1930s, the strategic situation in the Mediterranean had altered with the rise of Fascist Italy, which controlled the Greek-inhabited Dodecanese islands off the west coast of Turkey and had constructed fortifications on Rhodes, Leros and Kos. The Turks feared that Italy would seek to exploit access to the Straits to expand its power into Anatolia and the Black Sea region. There were also fears of Bulgarian rearmament. Although Turkey was not legally permitted to refortify the Straits, it nonetheless did so secretly.
In April 1935, the Turkish government dispatched a lengthy diplomatic note to the signatories of the Treaty of Lausanne proposing a conference to agree a new regime for the Straits and requested that the League of Nations authorise the reconstruction of the Dardanelles forts. In the note, Turkish Foreign Minister Tevfik Rüştü Aras explained that the international situation had changed greatly since 1923. Europe had then been moving towards disarmament and an international guarantee to defend the Straits. The Abyssinia Crisis of 1934–1935, the denunciation by Germany of the Treaty of Versailles and international moves towards rearmament meant that "the only guarantee intended to guard against the total insecurity of the Straits has just disappeared in its turn". Indeed, Aras said that "the Powers most closely concerned are proclaiming the existence of a threat of general conflagration". The key weaknesses of the present regime were that the machinery for collective guarantees were too slow and ineffective, and there was no contingency for a general threat of war and no provision for Turkey to defend itself. Turkey was therefore prepared to enter into negotiations with a view to concluding, in the near future, agreements for regulation of the Straits with essential security provisions for the inviolability of Turkey's territory, and also for continued development of commercial navigation between the Mediterranean and the Black Sea.
The response to the note was generally favourable; Australia, Bulgaria, France, Germany, Greece, Japan, Romania, the Soviet Union, Turkey, the United Kingdom and Yugoslavia attended negotiations at Montreux, Switzerland, which began on 22 June 1936. Two major powers were notably absent: Italy, whose aggressively expansionist policies had prompted the conference, refused to attend, and the increasingly isolationist United States declined even to send an observer.
Turkey, the United Kingdom and the Soviet Union each put forward their own set of proposals, each aimed chiefly at protecting the proponent's own interests. The British favoured the continuation of a relatively restrictive approach, the Turks sought a more liberal regime that reasserted their control over the Straits, and the Soviets proposed a regime that would guarantee absolute freedom of passage. The British, supported by France, sought to exclude the Soviet fleet from the Mediterranean Sea, where it might threaten the vital shipping lanes to India, Egypt and the Far East. In the end, the British conceded some of their requests, but the Soviets succeeded in ensuring that the Black Sea countries, including the Soviet Union, were given exemptions from the military restrictions imposed on non-Black Sea nations. The agreement was ratified by all of the conference attendees with the exception of Germany, which had not been a signatory to the Treaty of Lausanne, and with reservations by Japan, and came into force on 9 November 1936; it was registered with the League of Nations Treaty Series on 11 December 1936.
Britain's willingness to make concessions has been attributed to a desire to avoid Turkey being driven to ally itself with or to fall under the influence of Adolf Hitler or Benito Mussolini. It was thus the first in a series of steps by Britain and France to ensure that Turkey would either remain neutral or tilt towards the Western Allies in the event of any future conflict with the Axis.
As mentioned in its preamble, the Convention replaced the terms of the Lausanne Treaty of 1923 regarding the Straits. This had dictated the demilitarisation of the Greek islands of Lemnos and Samothrace, along with the demilitarisation of the Sea of Marmara and the Bosporus, the Dardanelles, and the Turkish islands of İmroz, Bozcaada and Tavşan.
The Convention consists of 29 Articles, four annexes and one protocol. Articles 2–7 consider the passage of merchant ships. Articles 8–22 consider the passage of war vessels. The key principle — freedom of passage and navigation — is laid out in articles 1 and 2. Article 1 provides, "The High Contracting Parties recognise and affirm the principle of freedom of passage and navigation by sea in the Straits". Article 2 states, "In time of peace, merchant vessels shall enjoy complete freedom of passage and navigation in the Straits, by day and by night, under any flag with any kind of cargo".
The International Straits Commission was abolished, thereby allowing the full resumption of Turkish military control over the Straits and the refortification of the Dardanelles. Turkey was authorised to close the Straits to all foreign warships during a war or when it was threatened by aggression. Also, Turkey was authorised to refuse transit from merchant ships belonging to countries at war with it.
A number of highly specific restrictions in Article 14 and 18 were imposed on what type of warships are allowed passage. Non-Black Sea powers wishing to send a vessel must notify Turkey 15 days prior to the requested passing, and Black Sea states must notify 8 days prior to passage. Also, no more than nine foreign warships, with a total aggregate tonnage of 15,000 tons, may pass at any one time. Furthermore, no single ship heavier than 10,000 tonnes can pass. An aggregate tonnage of all non-Black Sea warships in the Black Sea must be no more than 45,000 tons, with no one nation exceeding 30,000 tons at any given time, and they are permitted to stay in the Black Sea for at most 21 days. Only Black Sea states may transit capital ships of any tonnage, escorted by no more than two destroyers. Any revision to articles 14 and 18 requires 3/4 majority of signatory countries and must include Turkey.