This article is part of a series that will explore the use and legal issues surrounding military zones employed during peace and war to control the entry, exit, and activities of forces operating in these zones. These works build on the previous Maritime Operational Zones Manual published by the Stockton Center for International Law predecessor’s, the International Law Department, of the U.S. Naval War College. A new Maritime Operational Zones Manual is forthcoming.

By LtCol Brent Stricker

“We have attacked, fired upon, and dropped depth charges on a submarine operating in defensive sea area.”–USS Ward (DD-139) December 7, 1941, Pearl Harbor, Hawaii.

Submerged foreign submarines in a nation’s territorial sea pose a unique situation that is inconsistent with the rule of innocent passage. Under certain circumstances, their concealed presence without the consent of the coastal state could be considered a threat to the territorial integrity or political independence of the coastal state. A modern submarine fulfills its peacetime mission and combat role while submerged. If the coastal state detects a submerged submarine in the territorial sea, it is faced with a dilemma on the appropriate measures that can be used to force the submarine to surface or leave the territorial sea. The recent sabotage of the Nord Stream pipeline and the vulnerability of the world’s vast subsea network of electricity and network cables highlights the danger posed by unknown submersibles.

Norway and Sweden have faced this problem for more than 50 years from suspected Soviet and later Russian submarines. Both countries have used warning shots in an attempt to signal the submerged contacts to surface or leave the area. Use of explosives in this manner, however, could be misinterpreted as an attack on the submarine. Balancing the protection of territorial sovereignty with avoiding escalation poses a predicament.

Innocent Passage

All ships, including warships, enjoy the right of innocent passage through the territorial seas of a coastal state without prior notification or consent. This rule was discussed in detail in the Corfu Channel case before becoming codified in the United Nations Convention on the Law of the Sea. The Corfu Channel is a narrow passage between Albania and the Greek island of Corfu. The United Kingdom’s Royal Navy was confronted by Albanian coastal artillery fire when transiting the channel in May 1946. In October 1946, two Royal Navy destroyers transited the channel while at action stations to be prepared to respond to coastal artillery fire or other threat posed by the Albanians. These destroyers struck naval mines laid in the channel. As a result, in November 1946, the Royal Navy conducted minesweeping operations to clear the channel.

The United Kingdom brought a case against Albania in the International Court of Justice seeking reparations for the loss of life and damages to its warships. The ICJ upheld the Royal Navy’s right of innocent passage through Albanian territorial waters, rejecting Albania’s arguments that the ships were not in innocent passage because they were sailing in formation and the sailors on board were at action stations. Rather, the Court found that sailing in formation and running at action stations were appropriate defensive measures. The Court found that the minesweeping operation was inconsistent with innocent passage and a violation of Albanian sovereignty, rejecting the British arguments that this was a measure of “self-protection.” Corfu Channel illustrates how innocent passage may include defensive measures. The case has long presented a conundrum because it determined that states are entitled to innocent passage, yet are restrained from taking defensive action, such as minesweeping, to exercise their right.  

Innocent passage is governed by the United Nations Convention on the Law of the Sea (UNCLOS). Norway and Sweden are signatories to UNCLOS, and the United States, while not a signatory, recognizes much of it as customary international law. UNCLOS codified the right of innocent passage in Articles 17-21. Innocent passage must “not be prejudicial to the peace, good order, or security of the coastal state.” A foreign vessel’s passage is not innocent if its actions constitute “any threat or use of force against the sovereignty, territorial integrity or political independence of the coastal State.” A special provision for submarines, Article 20, requires submarines engaged in innocent passage to “navigate on the surface and to show their flag.”

A coastal state that discovers an unknown submerged contact in its territorial sea is faced with a dilemma. Examples from Norway and Sweden of submerged contacts lingering in their territorial waters are inconsistent with the definitions of both passage and innocent passage. The coastal state, under Article 25 of UNCLOS, may “take the necessary steps in its territorial sea to prevent passage which is not innocent.” There is no agreement on exactly what steps are deemed necessary. Furthermore, these measures are limited when applied to sovereign immune warships. Thus, while an unknown submerged contact is not exercising innocent passage, it is unclear what measures a coastal state can apply to exercise its rights under Article 25. Articles 30 and 31 of UNCLOS allow a coastal state to require the submerged contact to leave its territorial sea and places liability for any damages on the flag state of the submerged contact. Armed force against an unknown submerged contact, however, may only be used in self-defense under Article 51 of the UN Charter. In most cases, use of force would not be justified simply because the submarine is submerged or refuses to surface and the mere presence of the submarine does is not tantamount to an “armed attack.” This determination is complicated when the submerged contact’s intensions cannot be ascertained.

Norway

Norway has been dealing with suspected intrusions by foreign submarines for more than 50 years. These contacts in Norwegian fjords are difficult to track due to the mixing of fresh water runoff and salt water in the fjords which can provide cover for submarines from sonar detection. Acoustic detection is complicated by the fjord’s subsurface structure, currents, and civilian surface traffic. For two weeks in November 1972, Norwegian vessels aided by Norwegian and British aircraft attempted to locate and force to the surface an unknown underwater contact, believed to be of Soviet or Warsaw Pact origin, in the Sogne Fjord using depth charges. Hand grenades and then depth charges were used to signal to the underwater contact to surface. Ultimately, the Ministry of Defense was given permission to sink the contact if it did not surface and identify itself.

For the Norwegians to use force against the unknown submerged contact, they would need to articulate how an otherwise benign submerged vessel posed an imminent threat that would justify the use of force in self-defense. Violating Article 21 of UNCLOS in and of itself does not constitute such a threat of imminent attack, even if the submarine is engaged in an intelligence or reconnaissance mission. Such a mission may be illegal under Norway’s domestic law, but it does not imply an illegal use of force, let alone an armed attack.

In limited situations, the location and duration of the unknown submarine in territorial waters could be considered as a threat, as noted in the radio transmission of the USS Ward when it engaged an unknown submarine in a defensive sea area. The Norwegians would be more concerned by the location of the unknown submersible if it were in such an area or in close proximity to another sensitive military exercise or base. The longer the submarine remained at depth, the greater potential one might consider it laying in wait to attack. Nonetheless, the Norwegians employed an escalating use of force in 1972 with attempts to signal with hand grenades and ultimately culminating with firing anti-submarine missiles at the suspected target. The Norwegians were ultimately unable to force the contact to the surface, identify it, or sink it.

Norwegian experiences with unknown submarine contacts continued over the decades. The official Norwegian policy on the use of force remained somewhat ambiguous. In 1983, Brigadier Asbjorn V. Lerheim stated on the use of force, “It is a tough decision, it is still peacetime, and you can’t really destroy a submarine . . . it is not an attack on Norwegian soil.” Norway seems to have adopted a set of measures to escalate the use of force against these intrusions. The first measure is to signal the submarine to surface. If the submarine complies, it would be taken under escort. If not, depth charges would be dropped within 300 meters from the submarine with a two-minute interval to indicate this was a signaling measure, not an attack. If this failed to surface the submarine, Norwegian captains were authorized to attack with depth charges, but torpedoes were prohibited in the attack because of the potential of catastrophic damage to the boat and loss of the entire crew. It is speculated that the anti-submarine missiles fired in 1972 used homing devices and proximity fuses and were not a real attempt to hit the submarine.

Suspected Soviet incursions into Norwegian territorial waters continued as late as 1990. Norwegian authorities received reports of suspected submarines in the summer of 1990 at Skipton, a Norwegian bay twenty-five miles from the Russian border. The area was put under surveillance when, in November 1990, a mini-submarine was observed briefly on the surface. The sea floor was examined and a series of tracks were found that indicated a submersible crawler had been deployed. Similar tracks were discovered elsewhere in Sweden and Norway near military installations. The Soviet Northern Fleet possessed such miniature submarines at the time. It was speculated that the miniature submarine was launched from a nearby mother ship to conduct a Spetsnaz training or reconnaissance mission.

As late as 2021, Norway was subjected to an undersea intrusion by unknown submersibles. The Norwegian Institute of Marine Research operates a network of undersea sensors in northern Norway to monitor the marine environment. It can also be used to monitor submarines in the area. These sensors are interconnected by a series of fiber optic cables. In April 2021, it was discovered that 2.5 miles of fiber optic cable had been cut and stolen. Several of the sensors had been tampered with and moved. The reason for the intrusion is speculative but includes the potential for reverse engineering.

Sweden

Like Norway, Sweden has been troubled by intrusions of foreign submarines in its territorial waters for a similar period of time. Unlike Norway, Sweden has actually caught one submarine on the surface in the infamous “Whiskey on the Rocks” incident in 1981. This incident noted increased intrusions throughout the 1980s that have continued as late as the 2010s. To date, the Whiskey is the only foreign submarine caught on the surface in Swedish territorial waters.

On October 27, 1981, a Soviet Whiskey class submarine, the U-137, was found grounded on a rock in Swedish territorial waters. The Whiskey was an early Cold War diesel electric submarine, not a nuclear-powered submarine. The Swedish Navy contacted the submarine’s captain, Captain Second Rank A. M. Gushchin, who claimed a navigational error. Captain Gushchin claimed he thought he was 20 miles off the Polish coast when the collision occurred. This claim is rather dubious considering the submarine had transited submerged through a “perilous series of narrow straits infested with rocks and islands” before the grounding. The submarine’s grounding within ten kilometers of the Swedish naval base at Karlskrona while a major naval exercise was being conducted was certainly not just a coincidence brought about by a navigational error. Upon inspection, Swedish officials found no problems with the boat’s navigational equipment and noted its logbook had been altered.

The boat remained grounded for eleven days while the Swedish authorities inspected the submarine and questioned the captain. The Soviet Union responded by sending a flotilla of warships that stayed just outside Swedish waters. The Swedish Prime Minister made a shocking announcement on November 5, 1981, that the submarine was suspected of carrying nuclear weapons. The Swedish government made demands to the Soviets before releasing the submarine. However, weather intervened and Sweden released the submarine before these demands were met. The submarine was exposed to gale force winds and was listing 17 degrees. Swedish authorities were concerned that the boat’s battery acid could spill and cause a fire or release chlorine gas that could kill the crew. Swedish authorities stopped the captain’s interrogation and boat inspection, refloated the boat, and the submarine left on November 6, 1981.

Following this incident, the Swedish government released the Submarine Defense Commission Report in 1983, which detailed the history of foreign submarines intruding into Swedish waters. Prior to the Whiskey incident, and even subsequently, critics had claimed these submarine scares were an excuse to increase the Navy’s budget. The report detailed how foreign submarines entered Swedish waters typically one to two times a year in the 1970s before a dramatic increase during the 1980s. These incursions were concentrated around naval facilities such as coastal defense points, ports, sensor networks, and minefields.

The Report and increased submarine intrusions led to a change in Swedish Rules of Engagement (ROE) applicable to submarine contacts. Prior rules prohibited a commander from firing on an unknown contact without authorization from the civilian leadership. The Swedish Navy was only allowed to make contact with the submarine to identify it and escort it out of Swedish waters. The new ROE allowed the submarine to be fired upon without warning. Initially, warning shots were to be used, either through the employment of depth charges or missiles. The ROE were intended to prevent the damage or destruction of the submarine, but the ROE made a distinction on the location and behavior of the contact. If the submarine was located in Sweden’s outer waters, these are waters beyond the internal archipelago to the 12-mile limit, it would be warned and escorted out. If the submarine was found in internal waters, these are waters of Sweden’s internal archipelago, and refused to leave or proceeded further, it could be treated as hostile and force designed to damage or destroy the submarine could be used.

The Swedish ROE may have contributed to their inability to force submarines to the surface. If they employed depth charges or other devices with an eye toward avoiding damaging the detected submarines, the submarines could simply ignore these attempts. There is evidence that the Swedish ASW may have damaged a submarine. In the summer of 1988, eight pieces of unknown foreign submarine rescue equipment were recovered in the Stockholm archipelago. Similar equipment had been recovered in the 1970s and 80s.

The Swedish Navy continued to deal with foreign submarines intruding into Swedish waters throughout the 1980s. The government stopped providing statistics on these incursions in 1987. Subsequent reports have been vague in their descriptions. This may be to avoid highlighting their inability to stop or deter these incursions.

There is evidence that these incursions did occur. The Swedish Navy noted that these incursions have become more sophisticated with the use of multiple submarines, miniature submarines, and divers. The evidence for these incursions comes from sightings, sonar, and magnetic detection from Swedish sensor networks. There has also been evidence of keel marks and track marks on the sea floor similar to the Norwegian miniature submarine event noted above.

The miniature submarines may have also allowed military forces to surreptitiously land on Swedish territory. Between 3 to 6 March 1984, Swedish forces fired at swimmers on the island of Almo. The island was searched and food caches were located. The Swedes have also noted attacks on their “submarine nets, break-ins ashore, to the disruption and destruction of underwater mine lines.” In one case, they were blamed for the theft of a naval mine. Most shockingly, in 1985 fisherman pulled a drowned swimmer up in their nets. The nets had been placed illegally near a naval mine. It is presumed the diver was scouting the mine when he became entangled and drowned. The fisherman did not recover the body and abandoned their nets. When Swedish authorities investigated, the body had been cut out of the net and removed by unknown persons.

Conclusion

This historic submarine incursions remain relevant today, particularly considering heightened tensions from the Russian invasion of Ukraine and the recent application of Finland and Sweden to join the North Atlantic Treaty Organization. Much like the Norwegian fjords, the Swedish archipelago would be an area for these submarines to operate. The reasons for the incursions remain relevant today for any NATO-Russian conflict whether it be to conduct reconnaissance or the insertion of Special Forces. If there is a repeat of one of these Cold War examples such as a stranded submarine like the Whiskey, or more concerning, NATO forces hunting a submarine contact, the consequences could be manifold. First, NATO forces chasing a submarine contact trying to force it to surface might be viewed as an attack on the submarine. The use of explosives to signal a submarine might accidentally damage it or injure the crew. These signals could be misinterpreted as an attack allowing or even requiring a submarine to respond in self-defense. Second, any hostilities in territorial waters directly implicates the collective self-defense clause of Article 5 of the North Atlantic Treaty.

The conduct of Russian submarine espionage in the territorial seas of its neighbors presents one of the greatest challenges to avoiding conflict in the Baltic Sea. These incidents reveal the gap between the law of the sea and the use of force in self-defense against an armed attack. The Nordic coastal states must walk a fine line between protecting their territorial integrity and avoiding escalation of an incident that might quickly spin out of control.

LtCol Brent Stricker, U.S. Marine Corps, serves as the Director for Expeditionary Operations and as a military professor of international law at the Stockton Center for International Law, U.S. Naval War College. The views presented are those of the author and do not necessarily reflect the policy or position of the U.S. Marine Corps, the U.S. Navy, the Naval War College, or the Department of Defense.

Featured Image: Russian Kilo-class submarine in the English Channel. (UK Ministry of Defence photo via Wikimedia Commons)