Montreux Convention, Bosphorus Strait and the Making of Naval Power in the Black Sea
By Jeanne-May Desurmont | 6 November 2025
Summary
The Black Sea is a key theatre in the current Ukraine-Russia war. It is compounded by economic competition between the 2 parties, as it is the main route for their agricultural and oil exports. It also showcased the Ukrainian ability to shape naval combat through unmanned vessels and drone campaigns.
The 1936 Montreux Convention grants the control of the Black Sea straits to Türkiye. By limiting the access of warships in the Black Sea, Türkiye acts as a security guarantor and prevents naval escalation.
The current context presents an opportunity for Türkiye to emerge as a significant naval power in the region, as a NATO member, and to assert its influence.
Context
The Black Sea is a landlocked maritime area of approximately 440,000 km² and bounded by Russia, Ukraine, Bulgaria, Romania, Georgia and Türkiye. Connected to the Mediterranean Sea by the Bosphorus and the Dardanelles Straits, it is often overlooked in Western strategic thinking. However, it is as influential and as contentious as its Baltic and Arctic counterparts. Despite its lack of spotlight, the Black Sea is a key naval theatre in the Russia-Ukraine war and a vital chokepoint for international shipping and food trade. As the focal point of Russian southern power-projection, the Black Sea used to be the home of the Russian Black Sea Fleet which connected the Russian Navy to the Mediterranean and enabled the Kremlin to exert its influence in its regional sphere and the Middle East. For instance, the Black Sea Fleet served as the logistical hub for Russia's intervention in Syria. Following the invasion of Ukraine in February 2022, the Black Sea quickly became a focal point of the conflict. While initial analysis forecasted a Russian monopoly of the area, the Ukrainian drone and uncrewed surface vehicles warfare exposed the fundamental flaws of the Black Sea Fleet as its warships were forced to relocate from Sevastopol in Crimea to Novorossiysk (Russia). This included the sinking of symbolic Russian warships such as the cruiser Moskva. By leveraging technological innovations, Ukraine managed to impose a form of sea denial on Russia, shifting the power dynamics in the Black Sea. The weakened Russian Navy in the Black Sea presents an opportunity for other riparian nations to increase their naval presence in the region. Romania and Bulgaria have expanded their naval cooperation and operations, specifically with Türkiye in a joint task force to clear mines in the Black Sea and are currently modernising their maritime forces. Therefore, with Romania, Bulgaria and Türkiye as NATO members, the alliance is strengthening its presence in a historically Russian sphere of influence.
In addition to the naval struggle, the Black Sea is also a contest for international shipping and Russian and Ukrainian exports, as both are competing for the African and Middle Eastern markets. This competition for the MENA markets underscores the significance of the Black Sea as a vital route for global markets. In fact, access to these emerging and booming markets is a crucial pillar of Russian influence in the region. In peacetime, commercial vessels could navigate directly across the sea and through the Turkish Straits to the international shipping lanes. This route allowed the exportation of 2 pillar commodities: oil and grain. The importance of this route was highlighted as prices increased and forecasts predicted important food crises across the world, including famine warnings in Africa and the Middle East as Russian warships blocked Ukrainian commercial vessels. These disruptions reshaped global grain flows and forced EU countries to find alternative routes, such as increased reliance on railways, the Danube, and new suppliers. Similarly, the Black Sea is also a key route for Russian oil exports, as well as agricultural trade. Therefore, it is probable that in the event of a ceasefire, Russia will continue to obstruct Ukrainian commercial ships to undercut Ukrainian trade. However, as Ukraine regained control of the Black Sea, it managed to charter a new route for its vessels by navigating closer to the coasts of Romania and Bulgaria. Additionally, Ukrainian drones are currently targeting Russian oil hubs on the Black Sea coasts to sustain the current Ukrainian advantage in the area and force Russia to adapt its oil export networks.
The Black Sea remains a key theatre in the current Russia-Ukraine war, as it holds high strategic value in terms of both naval power and economic support/income for both warring parties. It is also a space for Türkiye to project its influence and its naval power. In fact, Türkiye controls the two straits that connect the Black Sea to the Mediterranean Sea and the international waters and could shape the outcome of the conflict as it is the main executor of the Montreux Convention.
Figure 1: Wheat exports through the Black Sea from Ukraine and Russia
The Montreux Convention
The Montreux Convention regarding the Regime of Straits, notably the Bosphorus and the Dardanelles straits, was signed in 1936 in Montreux, Switzerland, and authorised Türkiye to regulate the passage of civilian ships and warships into and out of the Black Sea. In peacetime, commercial and civilian vessels have complete freedom of passage under Montreux while military vessels have a limited right to passage. To enter the Black Sea, non-riparian navies must notify Turkish authorities as well as all members of the Convention, and also respect a certain tonnage limit; for example, aircraft carriers are not allowed to cross the straits. In wartime, if Türkiye is not involved in the conflict, it has the authority to block the passage of military vessels of the nations at war (Article 19), except warships returning to their homeport in the Black Sea while commercial vessels are free to cross the straits. However, if Türkiye is belligerent, it can take any decisions about the passage of warships as it sees fit.
Yet, the Montreux Convention doesn’t provide a clear definition of “war”, therefore it is at the discretion of Türkiye to apply Article 19. For example, the annexation of Crimea by “little green men” in 2014 did not trigger the clauses of the Montreux Convention. However, on 27 February 2022, the Turkish Foreign Ministry announced that Türkiye will exercise its rights under Montreux to close the straits to the ships from the nations at war, thereby qualifying the Russian invasion of Ukraine as a war. By deciding what constitutes “war”, Türkiye positions itself as a gatekeeper of the Black Sea’s security and militarisation, enhancing its diplomatic leverage with Russia and NATO. Additionally, Türkiye discouraged all warship transit regardless of whether they were belligerent or not and would consider any ship crossing the straits as a warring party. While this is not enshrined in the Montreux Convention, naval powers that might be tempted to assist or intervene in the Black Sea, such as NATO states, did not attempt to challenge Türkiye and the Montreux Convention. So far, both Ukraine and Russia have not objected to the Convention and Türkiye has only allowed warships that were rejoining their homeport to cross the Bosphorus Strait. As such, Ukraine was not able to enjoy the minehunters sent by the United Kingdom and the Netherlands and Russia cannot reinforce its naval capabilities in the Black Sea.
Bosphorus Strait, source: NASA
Türkiye as a Major Naval Power in the Black Sea
The global respect for Türkiye’s exercise of Montreux provides an interesting case for international law and the Western rule-based order, as no nation sought to challenge it. While initially designed to reduce the risk of conflict around the straits and act as a security guarantee for the riparian countries, it confers on Türkiye an important leverage and influence in the region. Ultimately, the Turkish position and interpretation of the Montreux Convention could tip the Ukraine-Russia war and favour one side. The legal text also enhances Türkiye’s power and influence in its own backyard, particularly given its significant stakes in the region, including rich natural gas resources and a key connectivity hub. This highlights the duality of Türkiye’s role as a regional stabiliser but also as an opportunistic power, as its control over the straits consolidates its naval presence and power. By curtailling a US-led or NATO initiative, and by profiting from the weakened Russian Black Sea Fleet, Türkiye is currently rising as the main naval power in the region. This is emphasised by the current demining operations in the Black Sea conducted by the Turkish navy alongside Romania and Bulgaria. This leads to three plausible but highly unlikely scenarios and a fourth that is highly likely.
Figure 2: Risk and Impact Matrix of Possible Scenarios of the Montreux Convention
Scenario 1: Status quo
Türkiye continues to apply Article 19 of Montreux and maintains an enhanced presence in the Black Sea, while the Russian navy is weakened and focuses its war effort on land and in the air. Then, Türkiye can become the major maritime actor in the region and reap the economic and energetic benefits of the Black Sea. Additionally, with the convention in place, Türkiye can grow as a maritime power without the destabilising presence of foreign navies such as the U.S.
Scenario 2: Türkiye negotiates with the EU
Türkiye uses its control of the straits to negotiate with the EU on key issues such as customs and market access. In return, Türkiye could allow EU warships to navigate the Black Sea and provide naval support to Ukraine. For example, it will allow the British and Dutch minehunters to support the Ukrainian Navy. However, this will likely undermine Türkiye’s neutrality and will strain its political relationship with Russia. This can also be perceived as escalatory behaviour from NATO members and increase instability in the region, as well as the risk of a broader naval conflict.
Scenario 3: Türkiye favours Russia
In this scenario, Russia attempts to regain a naval advantage and pressures Türkiye to open the straits to Russian warships. Given the strained Turkish relationship with the United States since the second Trump administration, Türkiye has decided to revoke its decision on Article 19 of the Montreux Convention. Türkiye can also be motivated to revoke Article 19 to gain leverage on energy and economic deals, as well as concessions on the purchase of Russian arms, such as the S-400s acquired in 2024. However, this also means that the straits are open to NATO navies, which could escalate the war into a naval conflict in the Black Sea, where the Russian navy is already weakened.
