Why does a 90-year-old treaty limit NATO to the Black Sea?

For nearly nine decades, a treaty has shaped the strategic dynamics of the Black Sea, limiting NATO’s military presence and influencing regional security. The Montreux Convention, signed exactly 90 years ago in Montreux, Switzerland, continues to govern access to the Bosporus and Dardanelles straits, two critical waterways that control entry into the Black Sea. These narrow passages, which separate Turkey from the rest of Europe, have long been central to geopolitical tensions, particularly since Russia’s invasion of Ukraine began four years ago. The Black Sea, bordered by six nations, Turkey, Bulgaria, Romania, Georgia, Ukraine, and Russia, is a focal point of European security. Of these, three are NATO members, while the other three are not, with two currently engaged in war. Understanding the region’s complexity requires careful attention to its geography. Access to the Black Sea is restricted to just one route: through the Turkish-controlled straits. At their narrowest point, the Bosporus is barely 700 meters wide, far narrower than the 33 kilometers of the Strait of Hormuz. This geographical bottleneck makes the Black Sea a chokepoint for both trade and military movement. Thomas de Waal, a senior researcher at the Carnegie Europe organization, described the situation as “somewhat paradoxical.” He noted that despite the Black Sea’s strategic importance, there is currently “relatively little activity involving combat vessels,” highlighting the unusual calm amid ongoing conflict. According to de Waal, the control of these straits gives Turkey a significant economic and military advantage. “Who controls the Turkish straits, to some extent controls trade through the Black Sea,” he explained. Darja Isačenko, a guest researcher at the German Institute for International and Security Affairs, added that Russia also has a vested interest in the Black Sea. She argued that Russia’s strong naval position here is not an end goal but a means to achieve broader geopolitical ambitions beyond the region and to reinforce its status as a great power. The Montreux Convention, adopted on July 20, 1936, was the result of negotiations among representatives of ten countries aimed at resolving who should manage these vital waterways. The agreement restored Turkish sovereignty over the Dardanelles and Bosporus after years of international oversight following the Ottoman Empire’s defeat in World War I. The convention established a legal framework for managing access to the straits, allowing free passage for civilian and commercial traffic during peacetime. During wartime, however, the rules become stricter. Combatant ships from nations involved in hostilities are barred from passing through, unless they originate from within the Black Sea region. The maximum stay for foreign warships is limited to 21 days, and their size and tonnage are restricted. During wartime, these provisions become even more stringent. If Turkey is not a party to the conflict, warships from any nation participating in the fighting are prohibited from passing through. This clause, enacted in February 2022, had profound implications for the region. It effectively placed Turkey in a powerful position, controlling the flow of military assets through the straits. Isačenko pointed out that Turkey’s influence is unmatched in the Black Sea. “The most privileged Black Sea country is Turkey itself,” she stated. This explains why NATO does not maintain a permanent presence in the Black Sea, unlike in the Mediterranean or North Seas. “For NATO, Montreux is a limitation,” she emphasized. “For Turkey, it is an instrument for maintaining balance in the Black Sea.” Russia’s invasion of Ukraine in late February 2022 posed the greatest challenge to the Montreux Convention in recent decades. Within days of the operation’s start, Turkey invoked Article 19 of the treaty, which applies during wartime and restricts the passage of combat vessels from warring states. As a result, neither Russia nor Ukraine could freely bolster their naval forces in the Black Sea, with one exception: warships based in the Black Sea could return to their home ports. De Waal argued that this move was crucial for Ukraine’s survival. “I believe the Montreux Convention and Turkey’s policy at the beginning of the war were absolutely key to Ukraine’s survival,” he told DW. He claimed it prevented Russia from significantly reinforcing its Black Sea fleet, noting that “around 30 Russian ships remained outside.” This, he suggested, contributed to Ukraine maintaining “advantage on the Black Sea.”