The GIUK gap is an area in the northern Atlantic Ocean that forms a naval choke point. Its name is an acronym for Greenland, Iceland, and the United Kingdom, the gap being the open ocean between these three landmasses. The term is typically used in relation to military topics. The area has been considered strategically important since the beginning of the 20th century.
The GIUK gap is particularly important to the Royal Navy, as any attempt by northern European forces to break into the open Atlantic would have to be made either through the heavily defended English Channel, one of the world's busiest seaways, or through one of the exits on either side of Iceland. As the British also control the strategic port of Gibraltar at the entrance to the Mediterranean, this means Spain, France, and Portugal are the only Continental European countries that possess direct access to the Atlantic Ocean that cannot easily be blocked at a choke point by the Royal Navy.
Since the beginning of the 20th century, the exploitation of the GIUK gap by northern forces and measures to patrol and secure the gap by opposing forces have played an important role in naval and in overall military planning.
From the start of World War II in 1939, German ships used the gap to break out from their bases in northern Germany (and from occupied Norway after April 1940) with a view to attacking Allied shipping convoys, but Allied blocking efforts in the North Sea and in the GIUK gap impeded such break-outs. British forces occupied the Faroe Islands in April 1940, and Iceland in May 1940; the United States took over effective control of Greenland in 1940. But the German Kriegsmarine profited greatly from the fall of France in June 1940, after which German submarines could operate from bases on the French coast. Between 1940 and 1942, the Denmark Strait between Iceland and Greenland remained one of the few areas that RAF patrol bombers could not reach, and thus became the centre for considerable action.
The origin of the term "gap" dates to this period, when there was a gap in air coverage known as the Mid-Atlantic gap or the "Greenland air gap". This gap was an area that land-based aircraft could not reach and where, as a result, they could not carry out their anti-submarine duties. The air-surveillance gap eventually closed in 1943, when longer-ranged versions of aircraft such as the Short Sunderland and B-24 Liberator came into service.
The GIUK gap again became the focus of naval planning in the 1950s, as it represented the only available outlet into the Atlantic Ocean for Soviet submarines operating from their bases on the Kola Peninsula. NATO worried that if the Cold War "turned hot", naval convoys reinforcing Europe from the U.S. would suffer unacceptable losses if Soviet submarines could operate in the North Atlantic. The United States and Britain based much of their post-war naval strategy on blocking the gap, installing a chain of underwater listening posts right across it during the 1950s – an example of a SOSUS "sound surveillance system". This deployment of sonar surveillance in the gap, and elsewhere, hampered the Soviet Northern Fleet's ability to deploy its submarines without detection.
The Royal Navy's primary mission during the Cold War, excluding its nuclear deterrent role, involved anti-submarine warfare (ASW). The development of the Invincible-class anti-submarine carriers stemmed from this doctrine: their primary mission involved anti-submarine warfare using Sea King helicopters. The Type 23 frigate originated as a pure ASW platform; its mission expanded following the Falklands War of 1982.
The Soviets planned to use the GIUK gap to intercept any NATO ships, especially aircraft carriers, heading towards the Soviet Union. Ships and submarines as well as Tupolev Tu-142 maritime-surveillance aircraft aimed to track any threatening ships.
The advent of longer-ranged Soviet submarine launched ballistic missiles (SLBMs) allowed the Soviet Navy to deploy their ballistic missile submarine (SSBNs) within protected bastions in the Barents Sea and reduced their need to transit the GIUK gap. The much reduced, post-Cold War Russian Navy has even less need to transit the GIUK gap.
Crossing the GIUK gap was a major strategic move for Ocean Venture in 1992, in which 84 NATO ships, including 4 US aircraft carriers, departed from their usual August exercise pattern, deployed a decoy south toward the mid-Atlantic, and then entered waters in a move that had historically been associated with a risk of destabilizing détente.
In late October 2019, a week before Commander of the Northern Fleet Aleksandr Moiseyev and Russian Foreign Minister Sergey Lavrov met their Norwegian counterparts in Kirkenes, Norway, ten submarines of the Russia's Northern Fleet, among them two diesel-electric and eight non-strategic nuclear, left their homebases in Kola Peninsula to participate in the largest Russian submarine drills since operations Aport and Atrina, in 1985 and 1987, respectively, when the Soviets deployed several SSNs near the U.S. coast before Gorbachev-Reagan meeting. The main task of the submarines was reportedly testing Russian ability to breach the GIUK gap undetected and sail into the Atlantic Ocean. The drills were expected to last up to two months.