Ninety Years of Montreux: Turkey, the Straits and the Balance of the Black Sea

In July 2026 the Montreux Convention turned ninety. Few treaties of the interwar era remain in force, and fewer still continue to shape the behaviour of great powers. Yet Montreux — which governs passage through the Bosphorus, the Sea of Marmara and the Dardanelles — remains one of the pillars of Turkish foreign policy and of the strategic balance in the Black Sea.

From Lausanne to Montreux#

The Treaty of Lausanne in 1923 had demilitarised the straits and placed them under an international commission. By the mid-1930s, with Italy expanding in the Mediterranean and the League of Nations visibly failing, Turkey sought to restore its full sovereignty. The conference at Montreux in 1936 granted it: Turkey regained the right to fortify the straits and to control their administration, in exchange for guaranteeing free passage under clearly defined rules.

The core rules#

The convention distinguishes sharply between merchant vessels and warships.

• Merchant ships enjoy complete freedom of transit and navigation in peacetime, whatever their flag or cargo.
• Warships must give advance notice and are subject to limits on class and tonnage.
• Non–Black Sea powers face a ceiling on the aggregate tonnage of warships they may keep in the Black Sea — 30,000 tons, which may rise to 45,000 in certain circumstances — and their vessels may stay no longer than 21 days.
• Black Sea states enjoy more generous terms, including the right to send capital ships through the straits under set conditions.

In wartime the balance tilts decisively towards Turkey. If Turkey is neutral, warships of belligerent states may not pass through the straits, except to return to their home bases. If Turkey is at war, or considers itself threatened by imminent danger of war, passage is left to its discretion.

Montreux in practice#

For most of the Cold War, the convention’s tonnage and duration limits helped keep the Black Sea a relatively closed strategic space, constraining both NATO and Soviet navies in different ways. Ankara has consistently defended the text against proposals to reinterpret or revise it, well aware that any renegotiation could erode its own position.

The convention’s wartime provisions were tested in February 2022, after Russia’s full-scale invasion of Ukraine. Turkey declared that the situation constituted a war and applied the relevant provisions, closing the straits to the warships of the belligerents, apart from vessels returning to their home ports, and urged other states not to send warships through. The decision was measured, legally grounded and — crucially — consistent with Turkey’s long-standing interest in being seen as the neutral custodian of the treaty.

Montreux allows Turkey to convert geography into diplomacy: the rules are fixed, but Ankara is the one who applies them.

The Canal Istanbul debate#

The government’s proposal, first announced in 2011, to build an artificial waterway parallel to the Bosphorus revived questions about the convention’s scope. Would ships using a new canal be subject to Montreux? Could the project be used to circumvent its limits? In April 2021 a public statement by more than a hundred retired Turkish admirals warning against any move that might call the convention into question provoked a sharp political reaction. The episode revealed how closely Montreux is bound up with Turkish ideas of sovereignty and national security.

Why it endures#

Montreux endures because it suits its most important party. It gives Turkey a recognised legal role at the junction of Europe and Asia, limits the ability of outside navies to change the balance in the Black Sea, and provides a framework that Russia, Ukraine and NATO members all have reasons to respect — even when they would prefer different rules. Ninety years on, it remains a reminder that some of the most consequential strategic arrangements in the region are written not in the language of force, but in the fine print of treaties.