The Danish Straits

There are three ways out of the Baltic Sea, and all of them run through Danish water. The Great Belt, the Øresund, and the narrow Little Belt are the only maritime exits from a sea ringed by nine states — and so they function, in the old phrase, as the cork in the Baltic bottle. For most of the past century that geographic fact was a Cold War footnote: the passages through which the Soviet Baltic Fleet had to squeeze to reach the open ocean. Today they are something more pointed — the route through which roughly 40 percent of Russia’s seaborne oil exports must pass, and therefore the place where the West’s sanctions on Russian energy meet the hard limits of the law of the sea.

Geographic Position¶

The straits thread between the Danish islands and the Scandinavian and Jutland mainlands, connecting the Baltic to the Kattegat and onward to the North Sea and Atlantic.

The Great Belt (Storebælt), between the islands of Zealand and Funen, is the principal deep-water route, dredged to take the largest tankers and spanned since 1998 by a fixed link whose suspension bridge carries one of the world’s longest main spans. The Øresund (the Sound), between Danish Zealand and Swedish Scania, narrows to about four kilometres at the historic crossing between Helsingør and Helsingborg, but a shallow sill restricts its draft — so the biggest ships use the Great Belt instead. The Øresund, crossed since 2000 by the Copenhagen–Malmö bridge, also forms the Denmark–Sweden maritime border. The Little Belt, between Funen and the Jutland mainland, is the narrowest and least commercially important. Together the straits carry on the order of 70,000 ship transits a year.

The Baltic’s Only Door¶

Like the Turkish Straits and the Strait of Hormuz, the Danish Straits owe their importance to the absence of alternatives. Everything that enters or leaves the Baltic by sea — Russian crude from the export terminals at Primorsk and Ust-Luga, Russian naval units based at Kaliningrad and St Petersburg, the container and bulk traffic of nine littoral states — must pass through one of these narrow lanes between Danish shores.

The energy dimension is the one that now dominates. The Danish Straits are among the world’s significant oil chokepoints: the US Energy Information Administration measured around 3.2 million barrels a day transiting in 2016, most of it Russian crude bound for Europe, and analyst estimates since the 2022 sanctions put the figure higher still — though those later numbers are less authoritative and should be read as estimates. When Russia opened the Primorsk terminal in 2005 and shifted its export weight from the Black Sea to the Baltic, it made these straits central to its energy revenues. That decision looks, in hindsight, like a strategic hostage to fortune.

The Sound Dues and the Free-Transit Trap¶

The Danes have been charging for this geography for a very long time. From 1429, the Crown levied the Sound Dues — every foreign ship passing the Øresund had to halt at Helsingør and pay a toll, a revenue stream that at its height supplied up to two-thirds of the Danish state’s income and built the castle of Kronborg, Shakespeare’s Elsinore. The tolls ended only in 1857, when the Copenhagen Convention abolished them in exchange for a lump-sum payment from the maritime powers and declared the straits free international waterways, with Denmark bound never to suspend the peaceful passage of merchant ships.

That five-century-old bargain is now the source of a thoroughly modern dilemma. Because Denmark treats the 1857 Convention as a governing treaty preserved under the UN Convention on the Law of the Sea, it cannot lawfully bar merchant traffic from the straits — which means it cannot simply close the door on the Russian tankers it would dearly like to stop. The toll-gate that once enriched Denmark has become a free-transit obligation that ties its hands.

The Shadow Fleet¶

That obligation collides with the central security problem of the 2020s Baltic: Russia’s “shadow fleet.” To keep its oil flowing past Western sanctions and the G7 price cap, Russia has assembled a fleet of ageing tankers with opaque ownership, no Western insurance, and a habit of switching off their transponders — and most of them sail through the Danish Straits. The Danish Maritime Authority counted hundreds of voyages by EU-sanctioned tankers through Danish waters in 2025, an almost daily procession of vessels that are uninsured, often poorly maintained, and a standing environmental hazard in some of Europe’s busiest and most enclosed waters.

The strategic stakes sharpened further after Finland joined NATO in 2023 and Sweden in 2024, turning the Baltic into something close to a NATO lake — with the Danish Straits as the alliance-controlled gateway. The same waters host a dense web of undersea cables and pipelines, and these have become targets. The Nord Stream pipelines were ruptured by underwater explosions in the Danish and Swedish economic zones in September 2022, an act whose attribution remains officially unresolved. In December 2024 a shadow-fleet tanker dragged its anchor across the Gulf of Finland, severing a power cable and several data lines; Finland boarded and seized it. In January 2025 NATO launched Baltic Sentry, deploying frigates, aircraft, and naval drones to guard the seabed infrastructure.

Denmark’s response has been shaped by the legal trap. Unable to blockade, it has turned instead to inspection — intensifying environmental and insurance checks on suspect tankers, deploying fuel-sniffing technology on the Great Belt bridge, and using the narrow authority that maritime law allows against vessels sailing under false or absent flags. By late 2025 Russia had begun escorting its shadow tankers with naval vessels, turning a commercial evasion into a quasi-military confrontation in NATO’s inland sea.

From Cold War Seam to NATO Lake¶

The straits’ military significance is older than the current crisis. Throughout the Cold War they were one of the most important seams in the European confrontation: the Soviet Baltic Fleet, based at Kaliningrad and Leningrad, had to pass through these narrows to reach the open Atlantic, and bottling it up there was a core mission of Danish and West German forces. NATO planners treated the straits as a gate they intended to hold shut in any war — the Baltic equivalent of the GIUK Gap that hemmed in the Soviet Northern Fleet. Control of the Danish narrows meant control of whether the Soviet Baltic Fleet was a Baltic problem or an Atlantic one.

The geopolitics of that gate have now swung decisively. The accession of Finland in 2023 and Sweden in 2024 means every state bordering the Baltic except Russia is a NATO member, and the sea is routinely described as a “NATO lake” — with the Danish Straits as its alliance-controlled exit. In strategic terms this gives NATO latent leverage over Russian maritime access that it has not held since before the Cold War, and it sharpens the irony of Denmark’s legal bind: the alliance commands the gate, yet the 1857 free-transit obligation forbids the gatekeeper from closing it to the very tankers funding Russia’s war.

That tension is now playing out in the seabed as much as on the surface. The dense lattice of power cables, data lines, and pipelines crossing these shallow, crowded waters has become a target for sabotage and “accidental” anchor-dragging, and NATO’s Baltic Sentry patrols are an attempt to police a vulnerability that free-transit rules make hard to pre-empt. The straits that once bottled up a fleet now guard the wires that light northern Europe.

Strategic Assessment¶

The Danish Straits are a case study in the gap between geographic leverage and legal freedom of action. Denmark sits astride the one route Russian oil must take, and yet a treaty from 1857 forbids it from simply shutting that route — a constraint Russia exploits with every uninsured tanker it sends through. The straits show how a chokepoint can be strategically decisive and legally untouchable at the same time, and why the contest over them is being fought with inspections, insurance rules, and seabed patrols rather than with blockades.

They also mark the return of the Baltic as a zone of confrontation. For a generation after the Cold War the sea was a quiet European backwater. The combination of Russian energy dependence on the straits, the shadow fleet, the sabotage of cables and pipelines, and the accession of Finland and Sweden to NATO has made it, once again, a front line — with the Danish narrows as its gate.

Conclusion¶

A medieval toll-gate has become a twenty-first-century pressure point. The Danish Straits carry the oil that funds Russia’s war and the cables that power northern Europe, and they do so under a free-transit rule that predates the automobile. That contradiction — between Denmark’s commanding position and its bound hands — is the essence of the modern crisis in the Baltic. The cork is in the bottle, but the law says Denmark may not push it home.

Sources & Further Reading¶

• “The shadow fleet is undermining the maritime order more brazenly than ever” (Atlantic Council) — on the scale of Russia’s sanctions-evading tanker fleet and the Baltic’s exposure.

• “The Danish and Turkish Straits are critical to Europe’s crude oil and petroleum trade” (US Energy Information Administration) — the authoritative data on oil flows through the straits.

• “The Regime of Passage Through the Danish Straits” by Alex G. Oude Elferink — the definitive legal study of the 1857 Convention and its modern implications.

• “The Influence of Sea Power upon History” by Alfred Thayer Mahan — on why command of narrow exits from enclosed seas has always conferred strategic advantage.