The Danish Straits

What it is

The Danish Straits are three waterways connecting the Baltic Sea to the Kattegat and onward to the North Sea: the Øresund (the Sound, between Denmark and Sweden), the Great Belt (Store Bælt, between the Danish islands of Funen and Zealand), and the Little Belt (Lille Bælt, between Funen and mainland Jutland). The Great Belt is the only deep-water route, with a navigable depth sufficient for fully laden large tankers and bulk carriers; it carries the bulk of heavy commercial traffic. The Øresund is shorter but depth-limited. Together, the three passages are the only marine exit from the Baltic Sea. Every maritime state on the Baltic rim, including Russia, Germany, Poland, the Baltic republics, Finland, and Sweden, routes seaborne exports and imports through them. Denmark is the gatekeeper state, flanking all three.

History

The straits' legal status was settled on 14 March 1857 with the Copenhagen Convention, which abolished the Sound Dues, a toll Denmark had levied on transiting vessels since the 15th century. In exchange for one-time lump-sum payments from maritime powers, including the United Kingdom, France, Prussia, and the Russian Empire, Denmark guaranteed free, unrestricted passage for all commercial vessels in perpetuity. The convention was reaffirmed in Article 282 of the 1919 Treaty of Versailles. When UNCLOS entered into force in 1994, Article 35(c) explicitly excluded from the transit-passage regime any strait governed by a "long-standing international convention"; Denmark's government holds that the 1857 treaty qualifies, meaning that UNCLOS Part III, which would give Denmark broader interdiction powers, does not apply. The legal architecture resembles that of the Turkish Straits under the 1936 Montreux Convention, though Denmark's ability to restrict warship transit is narrower still.

Current state

Since Russia's full-scale invasion of Ukraine in February 2022, the Danish Straits have become an active pressure point in the Western sanctions regime. EU-sanctioned tankers linked to Russia's shadow fleet made 292 voyages through Danish territorial waters in 2025, a rate of almost one per day, carrying crude oil and refined fuel from Russia's Baltic ports. Denmark tracks these transits but acknowledges it cannot legally block them: the 1857 Convention requires free passage for commercial vessels regardless of flag or ownership. Denmark's available tools are limited to safety and environmental inspections, and even these have rarely resulted in detention. Since 2024, six countries, Denmark, Sweden, Poland, Finland, Estonia, and the United Kingdom, have run coordinated inspection campaigns in the straits, the Kattegat, and the English Channel. The UK boarding of the Smyrtos in the English Channel in June 2026 signalled one enforcement escalation, while the French seizure of tankers in 2026 added legal precedent, though both relied on port-state jurisdiction rather than transit-passage interdiction in the straits themselves.

Relationships

The straits sit at the intersection of three tensions. First, NATO's Baltic posture: all five Baltic Sea member states plus Sweden and Finland are now NATO allies, meaning Russia's only Baltic Sea oil export corridor runs through waters flanked entirely by NATO members. Second, EU energy sanctions: the higher the number of listed shadow-fleet tankers, the more scrutiny Denmark faces about why they transit freely. Third, Baltic undersea infrastructure: the seabed approaches to the straits host major cables and pipelines; the destruction of Nord Stream in September 2022 and subsequent incidents have focused NATO attention on hybrid threats in this corridor.

What to watch

Three questions are live as of mid-2026. First, whether Denmark or a coalition partner will invoke port-state or safety-based grounds to detain a shadow-fleet tanker while in transit, which would directly test the 1857 Convention's open-passage guarantee and likely trigger litigation. Second, whether the pace of EU sanctions listings, roughly 600 tankers designated by early 2026, will outrun the enforcement capacity of the six-country inspection coalition. Third, how far NATO's Baltic Sea patrol mission, endorsed at the 2024 Washington Summit, will extend: current mandates are observational, but political pressure for active interdiction is rising across Baltic-rim capitals.