Annex II Excludes Aircraft Carriers From Capital Ships: Why the Montreux Convention Bars Modern Carriers From the Turkish Straits

The Bosporus and Dardanelles straits form the only maritime passage connecting the Mediterranean to the Black Sea, serving as a critical artery for global trade. Control over this strategic chokepoint has shaped Eurasian geopolitics for centuries, driving conflicts between the Russian and Ottoman empires. Today, transit through these waters is governed entirely by a single document signed in Switzerland nearly a century ago.[5]

Before 1936, the straits were demilitarized and managed by an international commission under the 1923 Treaty of Lausanne. As global tensions rose in the 1930s, the Turkish government successfully argued that it needed to remilitarize the zone to ensure its national defense. The resulting negotiations produced a framework that balanced Turkish sovereignty with Soviet security concerns.[5]

The 1936 Montreux Convention Regarding the Regime of the Straits restored Turkish military control over the waterways while establishing a rigid legal framework for naval movement. The treaty guarantees complete freedom of passage for civilian merchant vessels during peacetime. However, it imposes severe restrictions on the number, type, and weight of military ships allowed to enter the Black Sea.[1][5]

These restrictions are designed to prevent a sudden naval buildup that could destabilize the region. The convention distinguishes sharply between Black Sea littoral states, such as Russia and Turkey, and outside nations. For non-Black Sea powers, the treaty caps the aggregate tonnage of their warships in the sea at 45,000 tons at any given time.[3]

Furthermore, no single vessel from a non-littoral state may exceed 15,000 tons, and these ships cannot remain in the Black Sea for more than twenty-one days. The treaty also mandates strict notification protocols for any naval transit. Non-Black Sea powers must notify Turkish authorities fifteen days before a planned passage, while littoral states need only provide eight days' notice.[3]

The Capital Ship Exemption

To accommodate the Soviet Union, the Montreux Convention included a specific exemption for the major fleets of Black Sea states. Article 11 allows these nations to send "capital ships" of any tonnage through the straits. This permits large battleships and cruisers to transit, provided they pass singly and are escorted by no more than two destroyers.[1][3]

This exemption is the only legal mechanism that allows warships heavier than 15,000 tons to navigate the Bosporus. For decades, it enabled the Soviet Navy to move its heaviest surface combatants between its Black Sea shipyards and the open ocean. However, the treaty's authors carefully defined what qualifies for this privilege to prevent abuse.[1]

Annex II of the convention provides the technical definitions for every category of warship. It defines capital ships primarily by their displacement and the caliber of their main guns. Crucially, the annex explicitly excludes aircraft carriers from the capital ship category, placing them in a distinct classification of their own with separate operational rules.[1]

The annex also addresses submarines, which are generally barred from transiting the straits to prevent covert operations. Black Sea powers are granted a narrow exception to move submarines through the waterways only if they were constructed or purchased outside the Black Sea, or if they require external repairs. Even then, they must travel by day and on the surface.[1]

Defining the Aircraft Carrier

As outlined in the official text of the treaty, Annex II defines an aircraft carrier as a "surface vessel of war, whatever its displacement, designed or adapted primarily for the purpose of carrying and operating aircraft at sea." The treaty notes that fitting a landing deck on a warship does not automatically make it a carrier. The vessel's primary purpose must be aviation.[1]

Because aircraft carriers are excluded from the definition of capital ships, they cannot utilize the Article 11 exemption. Black Sea powers have no special right to transit carriers exceeding the standard 15,000-ton limit. For non-Black Sea powers, the absolute 15,000-ton ceiling applies to all vessels regardless of classification, creating an impenetrable legal barrier.[1][3]

This legal architecture effectively bans modern aircraft carriers from the Turkish Straits. A contemporary United States Navy supercarrier, such as those in the Nimitz class, displaces approximately 100,000 tons at full load. This massive size makes it legally impossible for American carrier strike groups to enter the Black Sea under any circumstances, preserving the basin's isolation.[4]

Even lighter allied carriers, such as the United Kingdom's Queen Elizabeth class or France's Charles de Gaulle, far exceed the 15,000-ton limit. Consequently, the primary power projection tool of Western navies is entirely locked out of the basin. This restriction fundamentally alters how NATO plans and executes maritime operations in the region, forcing reliance on land-based airpower.[4]

The Soviet Aviation Cruiser Workaround

During the Cold War, the Soviet Union faced a unique strategic dilemma created by these rigid rules. Its only shipyard capable of constructing large aircraft-carrying vessels was located in Mykolaiv, on the coast of the Black Sea. To deploy these ships to the Northern or Pacific fleets, they had to pass through the Turkish Straits.[2]

To bypass the Montreux restrictions, Soviet naval architects designed a hybrid class of warships. Vessels like the Kiev and the later Admiral Kuznetsov were equipped with angled flight decks and arresting gear for fixed-wing aircraft. However, they were also armed with massive batteries of offensive anti-ship missiles, blurring the lines of traditional naval classification.[2]

The Admiral Kuznetsov carries twelve P-700 Granit cruise missiles in vertical launch tubes beneath its forward deck. This heavy surface-strike armament gave Moscow a factual basis to argue that aviation was not the ship's sole primary purpose. The Soviet Union officially designated the vessel as a "heavy aviation cruiser" rather than an aircraft carrier.[2]

The cruiser also carried a dense defensive weapons suite, including Kinzhal surface-to-air missiles and Kashtan close-in weapon systems. This robust organic firepower contrasted sharply with Western carriers, which rely heavily on escorting destroyers for protection. The resulting ship was part carrier and part heavily armed surface combatant, tailored specifically to exploit the treaty's language.[2]

Turkey accepted this classification, allowing the ships to transit the straits as capital ships under Article 11. Legal specialists debated whether the full-length flight deck should have triggered the aircraft carrier definition. However, no international court ever ruled on the issue, and the designation worked in practice, allowing the Soviet Navy to bypass the tonnage limits.[2]

Preserving the Regional Balance

The aviation cruiser workaround allowed the Soviet Union to comply with the letter of the convention while achieving its strategic goals. It permitted Moscow to extract its largest warships from the Black Sea shipyards. At the same time, it preserved the treaty framework that kept NATO aircraft carriers out of the region, maintaining a favorable balance of power.[7]

Western navies rely on aircraft carriers as their primary instruments of power projection. Unlike the heavily armed Kuznetsov, American carriers depend almost entirely on their embarked air wings and escorting destroyers for both offense and defense. They fit the Montreux definition of an aircraft carrier perfectly and remain barred from transit, limiting their operational reach.[2]

This dynamic has profoundly shaped the naval balance in the Black Sea during modern conflicts. During the 2008 Russo-Georgian War and the ongoing Russian invasion of Ukraine, the United States has been unable to deploy carrier-based airpower into the basin. NATO presence is restricted to lighter vessels like destroyers and frigates, which lack the strike capacity of a carrier wing.[3][5]

The tonnage limits for non-Black Sea powers remain strictly enforced by Turkish authorities. A typical American Arleigh Burke-class destroyer displaces around 9,000 tons, well under the 15,000-ton single-ship limit. However, a task force of just four such destroyers would approach the 45,000-ton aggregate ceiling for all non-littoral forces combined, severely limiting the size of any allied deployment.[3]

The treaty does include a minor provision for humanitarian missions. Non-Black Sea powers can send a naval force of up to 8,000 tons into the sea for humanitarian purposes without the standard fifteen-day notification. However, this small allowance offers no meaningful loophole for combat operations or carrier deployments, ensuring the basin remains insulated from sudden escalations.[3]

Wartime Provisions and Modern Enforcement

The Montreux Convention also grants Turkey significant authority during times of conflict. Under Article 19, if Turkey is not a belligerent in a war, warships of the warring nations are prohibited from passing through the straits. The only exception is for vessels returning to their registered home bases, preventing fleets from using the straits to reinforce active combat zones.[1][5]

Following the Russian invasion of Ukraine, Turkey faced immediate diplomatic pressure. "We are calling for the airspace, Bosphorus and Dardanelles Straits to be closed," Ukrainian Ambassador to Türkiye Vasyl Bodnar stated during a press conference in Ankara. Shortly after, Turkey formally invoked Article 19, recognizing the conflict as a war and closing the straits to belligerent warships.[6]

This decision effectively closed the straits to both Russian and Ukrainian naval reinforcements. It prevented Moscow from moving additional surface combatants from its Mediterranean squadron into the Black Sea to replace combat losses. The strict enforcement of these wartime provisions underscores Turkey's commitment to upholding the treaty's original intent.[5][6]

Despite periodic calls for revision, the Montreux Convention has proven remarkably durable. Turkey has consistently defended the treaty, viewing it as a vital guarantor of its own security and sovereignty over the straits. Opening the agreement to renegotiation could expose Ankara to pressure from both Russia and the West, risking the delicate balance it currently maintains.[5][7]

As naval technology evolves, the rigid definitions of Annex II continue to dictate fleet movements. The treaty ensures that the Black Sea remains a unique maritime environment, insulated from the supercarriers that dominate the open oceans. The diplomatic compromise struck in Switzerland nearly a century ago still commands the waters of the Turkish Straits today.[7]