Cover Image Source: The Arleigh Burke-class guided-missile destroyer USS Donald Cook (DDG 75) transits the Bosphorus Strait, Jan. 28, 2019. (U.S. Navy photo by Mass Communication Specialist 2nd Class Ford Williams/Released)
Author’s Note: The Turkish Straits and the Black Sea are included in the Strategic Waterway Under Pressure series because they raise a different question from those posed by Hormuz or Malacca. The issue here is not simply whether crisis is squeezing passage, but how a special treaty regime structures access when armed conflict reshapes the security environment of the Black Sea region.
The Turkish Straits and the Black Sea require a different analytical vocabulary from the one used for Hormuz or Malacca. The key question is not whether the Montreux Convention makes passage conditional. In important respects, it has always done so. The more consequential issue is how war in the Black Sea has affected the practical value of a formally open but legally differentiated regime of access.
The answer is necessarily qualified. The Montreux Convention Regarding the Regime of the Straits does not create a single, undifferentiated right of passage. It distinguishes between merchant vessels and warships, between Black Sea and non-Black Sea Powers, and between peace, war, and imminent danger of war. That original differentiation is not a weakness of the regime; it is its legal design. Yet the continued legal openness of the Straits, especially for merchant shipping, should not obscure a second development: the operational environment of the Black Sea now imposes burdens better described in commercial and security terms. Ships may still pass, but under conditions that are less insurable, less predictable, and less secure than the treaty text alone would suggest. The Turkish Straits therefore illustrate a complex form of conditionality: not the collapse of legal order, but the interaction between treaty-based differentiation and corridor degradation in practice.
The Turkish Straits remain one of the world’s most consequential maritime chokepoints. According to the U.S. Energy Information Administration, an estimated 3.7 million barrels per day of crude oil and petroleum products flowed through the Turkish Straits in the first half of 2025, equivalent to roughly 5 per cent of global maritime oil trade. Turkish official reporting also underlines the density of traffic, the presence of hazardous cargoes, and the navigational risks produced by currents, sharp turns, and proximity to densely populated urban areas.
Their strategic significance lies not only in the traffic they carry, but in the basin they serve. The Straits are the maritime outlet of the Black Sea, a major corridor for oil, grain, and other bulk trade. The Black Sea Initiative demonstrated that shipping through this basin can have consequences beyond the region itself, including for food supplies and price stability. The Turkish Straits are therefore not merely narrow waterways under Turkish sovereignty. They are the legal and operational hinge between the Black Sea and wider maritime circulation.
For that reason, the relevant problem is not well described as a simple question of closure. The Straits have not become a classic case of suspended merchant passage. The more difficult question is whether a corridor can remain formally open at its narrowest point while becoming more fragmented, more heavily risk-laden, and less predictable as a route system. That is the form in which conditionality arises here.
The starting point is that the Turkish Straits are not governed, in the first instance, by the default regime of transit passage under Part III of the United Nations Convention on the Law of the Sea. Article 35(c) of UNCLOS provides that Part III does not affect “the legal regime in straits in which passage is regulated in whole or in part by long-standing international conventions in force specifically relating to such straits.” The Turkish Straits are the clearest example of such a special regime. Their governing framework is the Montreux Convention as a lex specialis arrangement.
This remains true notwithstanding the fact that Türkiye is not a party to UNCLOS. The legal point does not depend on Turkish participation in UNCLOS. It depends on the continuing force of Montreux and on UNCLOS itself preserving long-standing special conventions of this kind. The Turkish case therefore differs from Hormuz in an important respect: the principal question is not whether a non-party may resist the ordinary straits regime, but how a special straits regime, expressly built around differentiated access, operates under contemporary strategic pressure.
The object and structure of Montreux are revealing. Its preamble states that the Convention regulates passage and navigation in the Straits of the Dardanelles, the Sea of Marmara, and the Bosphorus in a manner designed to safeguard Turkish security and the security of the Black Sea riparian States while preserving the principle of passage and navigation. The legal design is therefore dual from the outset: openness is affirmed, but it is not absolute. It is mediated through security-sensitive distinctions written into the treaty itself.
For merchant vessels, the baseline remains comparatively liberal. Article 2 of the Montreux Convention provides that, in time of peace, merchant vessels enjoy complete freedom of passage and navigation in the Straits, by day and by night, under any flag and with any kind of cargo, without formalities other than those specified in the Convention. No taxes or charges may be levied except those authorized by Annex I. Article 4 extends that logic into wartime when Türkiye is not itself a belligerent: merchant vessels, under any flag and with any kind of cargo, continue to enjoy freedom of passage and navigation subject to Articles 2 and 3, and pilotage and towage remain optional.
At the same time, Montreux does not reduce merchant navigation to an unrestricted laissez-faire model. Article 5 applies when Türkiye is a belligerent: merchant vessels not belonging to a State at war with Türkiye retain freedom of passage only on condition that they do not assist the enemy, and their passage must take place by day along routes indicated by Turkish authorities. Article 6 applies when Türkiye considers itself threatened with imminent danger of war: Article 2 continues to apply, but entry must be by day, routing may be directed by Turkish authorities, and pilotage may be made obligatory without charge. Even for commerce, then, Montreux creates a graduated regime in which openness is preserved but may tighten in defined security contingencies.
That point matters conceptually. Some forms of conditional access are embedded in the governing instrument itself. The sharper legal question is whether qualification remains within the treaty structure or hardens into something more discretionary than the treaty permits. The significance of the warship provisions, for present purposes, is not that this article is chiefly about naval transit. They make that legal design most visible.
The differentiated character of the regime is more pronounced in relation to warships. Here Montreux is not simply a passage convention; it is a security convention. The treaty distinguishes among categories of naval vessels, imposes prior notification requirements, limits aggregate tonnage in transit, places caps on the presence of non-Black Sea naval forces in the Black Sea, and restricts the duration of their stay there. Article 13 requires diplomatic notification before warship passage. Article 14 limits the maximum aggregate tonnage of foreign naval forces in transit through the Straits to 15,000 tons, with specified exceptions, and provides that such forces may not comprise more than nine vessels. Article 18 limits the aggregate tonnage of non-Black Sea Powers in the Black Sea, provides for a ceiling that may rise from 30,000 to 45,000 tons in specified circumstances, limits any one non-Black Sea Power to two-thirds of the aggregate limit, and provides that warships of non-Black Sea Powers may not remain in the Black Sea for more than twenty-one days.
These rules do more than regulate traffic. They structure the regional naval balance. In that sense, the Turkish Straits are not merely a commercial access point with an ancillary military dimension. They are a waterway in which access has long been legally segmented according to strategic category. It is therefore analytically misleading to describe the present case simply as freedom of navigation under strain. The more precise issue is the operation of a treaty regime drafted precisely to prevent the Straits from functioning as a fully open naval corridor on general law-of-the-sea terms.
That architecture becomes most visible in wartime. Article 19 applies when Türkiye is not a belligerent. In that situation, warships of non-belligerent States retain freedom of passage and navigation through the Straits under the conditions laid down in Articles 10 to 18. By contrast, warships belonging to belligerent Powers may not pass through the Straits, subject to limited treaty exceptions, including certain cases connected to Article 25, assistance to a State victim of aggression under a qualifying mutual-assistance treaty, and the right of warships separated from their bases to return to them.
This provision moved from dormant background law to active strategic significance in 2022. Following Russia’s invasion of Ukraine, Türkiye treated the conflict as a war for purposes of Montreux and applied Article 19 to restrict the passage of warships belonging to the belligerent Powers, while recognizing the treaty-based exception for return to base. That step was legally consequential, but not legally anomalous. It did not amount to an extra-conventional suspension of passage. It was an application of Montreux’s own wartime logic.
This is the point at which precision matters. Article 19 does not authorize a general closure of the Straits to all warships when Türkiye is not a belligerent. It restricts the warships of belligerent Powers, while preserving the passage of non-belligerent warships under the ordinary Montreux conditions. Articles 20 and 21 provide broader Turkish discretion only where Türkiye is itself a belligerent, or where it considers itself threatened with imminent danger of war. The Turkish Straits therefore differ sharply from settings in which a coastal State seeks to superimpose restrictive conditions on an otherwise general right of passage. Here the segmentation of access is internal to the governing regime itself.
Yet this still leaves the core question for the present series: whether formal openness in the Straits translates into meaningful openness for the corridor as a whole. The answer is increasingly uncertain. The legal regime of merchant passage remains in place, but the practical burdens of using the Black Sea route have grown as the war has spread across the basin’s shipping environment.
The IMO statement of 16 December 2025 put the point plainly. The Secretary-General called on all parties to refrain from targeting innocent seafarers, port workers, and merchant ships, and warned of mounting environmental risk as the situation in the Black Sea escalated. That statement widens the inquiry from the legal status of the Straits to the safety of the maritime system connected to them. The question is no longer only whether ships may lawfully pass the Bosphorus and the Dardanelles. It is whether the basin to which those Straits provide access remains usable on commercially and physically tolerable terms.
Official Turkish statements from late 2025 to early 2026 reinforce that conclusion. On 29 November 2025, Türkiye’s Ministry of Foreign Affairs expressed concern over attacks on two commercial tankers in the Black Sea and stressed the resulting risks to navigation, life, property, and the environment. On 12 December 2025, it stated that the attack on Ukraine’s Chornomorsk Port, which damaged a foreign-flagged vessel owned by a Turkish company, confirmed Turkish concerns about the spread of the war to the Black Sea and its impact on maritime security and freedom of navigation. On 26 March 2026, it expressed concern over the attack on the Sierra Leone-flagged, Turkish-operated tanker ALTURA in the Black Sea, again emphasizing risks to life, property, navigation, and environmental safety.
Taken together, these statements show two things. First, the problem is no longer confined to naval access or the abstract operation of Article 19; merchant shipping itself has become exposed to the spillover effects of war. Second, they clarify the distinction between formal legal openness and practical corridor usability. The issue is not simply that the Black Sea has become dangerous, but that danger now functions as a practical filter on access. Passage remains lawful, yet increasingly contingent on insurability, routing tolerance, port exposure, and conflict-related commercial risk. In that sense, the relevant conditionality does not arise from Türkiye’s conversion of passage into a permission-based regime. It arises from the interaction between a formally open strait and an increasingly hazardous basin.
The Turkish Straits and the Black Sea add an important qualification to the argument advanced across this series. Conditional passage is not always best understood as the product of overt legal restriction. It may also emerge where a waterway is governed by a lawful but differentiated regime and where the wider security environment intensifies the burdens of using the route that the regime keeps formally open.
The broader implication is that openness in strategic waterways is not determined by treaty text alone. In some settings, the decisive variable is the formal scope of navigational entitlement. In others, it is the interaction between legal form, security risk, commercial viability, and the condition of the basin beyond the narrow waterway itself. Conditional passage, on this view, is not only a matter of legal restriction; it is also a change in the effective terms of access. The Turkish Straits and the Black Sea bring that point into sharp focus.
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