Why a 90-year-old treaty limits NATO in the Black Sea

What Happened

The Montreux Convention of 1936 grants Turkey sovereignty over the Dardanelles and Bosporus straits, the sole maritime entry to the Black Sea. In peacetime, the treaty restricts non-Black Sea navies' warship tonnage and limits their stays to 21 days. During war, when Turkey is not a belligerent, warships of combatant nations are barred from transiting. After Russia's full-scale invasion of Ukraine on 24 February 2022, Turkey invoked Article 19 of the convention, preventing both Russian and Ukrainian warships from reinforcing their Black Sea fleets, with an exception for vessels returning to home ports. Thomas de Waal of Carnegie Europe told DW that this move 'was absolutely crucial in allowing Ukraine to survive,' estimating that up to 30 Russian ships remained outside the Black Sea. The UK Ministry of Defence confirmed that two British-donated minesweepers were unable to enter the Black Sea because Turkey classified them as 'vessels of war,' leaving them docked in Portsmouth. Meanwhile, Ukraine shifted to drones and other technologies not covered by the 1936 treaty, altering the naval dynamic.

Key Actors

Why It Matters

The Montreux Convention remains a cornerstone of Black Sea security, enabling Turkey to balance regional powers and limiting NATO’s direct naval involvement. It shaped the course of the war by denying Russia reinforcement, contributing to Ukraine’s naval advantage, and forcing both sides to innovate. Its enduring relevance, accepted even by Moscow, underscores how a 90-year-old treaty continues to influence modern warfare and alliance dynamics in a critical economic and military chokepoint.

Watch For

Monitor any attempts by Russia or NATO to challenge or reinterpret the convention, Turkish policy shifts (e.g., relaxation or tightening of transit rules), the impact of unmanned systems and new technology on maritime access regimes, and diplomatic discussions on Black Sea security architecture, especially if ceasefire or peace negotiations arise.