Operation Gearbox II (17 September 1942 – 7 September 1943) was a Norwegian and British operation during the Second World War on the Arctic island of Spitsbergen in the Svalbard Archipelago. Operation Fritham, the first attempt to establish a base had been defeated when the two ships carrying the force were sunk by Luftwaffe bombers on 14 May.

In Operation Gearbox (30 June – 17 September 1942), 57 Norwegian reinforcements with 116 long tons (118 t) of supplies had arrived by cruiser on 2 July. The reinforcements consolidated the defences of the former settlement at Barentsburg and made preparations for Gearbox II, another reinforcement of the Norwegians and part of the plan for Convoy PQ 18, to prevent a repeat of Convoy PQ 17 (27 June – 10 July 1942) in which 24 of the 35 freighters in the convoy had been sunk.

Force P (the Spitsbergen Fuelling Force) comprising the fleet oilers RFA Blue Ranger and RFA Oligarch and four destroyer escorts, sailed from Scapa Flow on 3 September and anchored in Lowe Sound (Van Mijenfjorden) several days later. From 9 to 13 September, relays of destroyers were detached from PQ 18 to refuel before the convoy passed Bjørnøya (Bear Island), into range of Luftwaffe bombers and torpedo-bombers based in north Norway.

After another German weather party was chased off the island by the Norwegians in June 1943, a German flotilla, including Tirpitz, raided Spitsbergen in Operation Zitronella on 7 September, took 31 prisoners and destroyed much of the infrastructure and equipment of Gearbox II. On 19 October, the cruiser USS Tuscaloosa and four destroyers delivered more Norwegian troops.

The Svalbard Archipelago is in the Arctic Ocean, 650 mi (1,050 km) from the North Pole. The islands are mountainous with permanently snow-covered peaks, some glaciated; there are occasional river terraces at the bottom of steep valleys and some coastal plains. In winter, the islands are covered in snow and the bays ice over. Spitsbergen Island has several large fiords along its west coast and Isfjorden is up to 10 mi (16 km) wide. The Gulf Stream warms the waters and the sea is ice-free during the summer. Settlements were established at Longyearbyen and Barentsburg in inlets along the south shore of Isfjorden, in Kings Bay (Quade Hock) further north along the coast and in Van Mijenfjorden (Lowe Sound) to the south. The settlements attracted colonists of different nationalities and the Svalbard Treaty of 1920 neutralised the islands and recognised the mineral and fishing rights of the participating countries. Before 1939, the population consisted of about 3,000, mostly Norwegian and Russian, people who worked in the mining industry. Drift mines were linked to the shore by overhead cable tracks or rails and coal dumped over the winter was collected by ship after the summer thaw. By 1939, production was about 500,000 long tons (510,000 t) a year, split between Norway and Russia.

The British Government Code and Cypher School (GC&CS) based at Bletchley Park housed a small industry of code-breakers and traffic analysts. By June 1941, the German Enigma machine Home Waters (Heimish) settings used by surface ships and U-boats could quickly be read. On 1 February 1942, the Enigma machines used in U-boats in the Atlantic and Mediterranean were changed but German ships and the U-boats in Arctic waters continued with the older Heimish (Hydra from 1942, Dolphin to the British). By mid-1941, British Y-stations were able to receive and read Luftwaffe W/T transmissions and give advance warning of Luftwaffe operations. In 1941, interception parties code-named headaches were embarked on warships and from May 1942, computers sailed with the cruiser admirals in command of convoy escorts, to read Luftwaffe W/T signals which could not be intercepted by stations in Britain. The Admiralty sent details of Luftwaffe wireless frequencies, call signs and the daily local codes to the computers. Combined with their knowledge of Luftwaffe procedures, the computers could give fairly accurate details of German reconnaissance sorties and sometimes predicted attacks twenty minutes before they were detected by radar. In February 1942, the German Beobachtungsdienst (B-Dienst, Observation Service) of the Kriegsmarine Marinenachrichtendienst (MND, Naval Intelligence Service) broke the British Naval Cypher No 3 and was able to read it until January 1943.

The Germans left the Svalbard islands alone during the invasion of Norway in 1940 and apart from a few Norwegians taking passage on Allied ships, little changed; wireless stations on the islands continued to broadcast weather reports in clear, which were useful to both sides. From 25 July to 9 August 1940, the cruiser Admiral Hipper sailed from Trondheim to search the area from Tromsø to Bear Island (Bjørnøya) and Svalbard, to intercept British ships returning from Petsamo but found only a Finnish freighter. On 12 July 1941, the Admiralty was ordered to assemble a force of ships to operate in the Arctic in co-operation with the USSR, despite objections from Admiral John (Jack) Tovey, commander of the Home Fleet, who preferred to operate further south, where there were more targets and better air cover.

Rear-admirals Philip Vian and Geoffrey Miles flew to Polyarnoe and Miles established a British military mission in Moscow. Vian reported that Murmansk was close to German-held territory, that its air defences were inadequate and that the prospects of offensive operations on German shipping were poor. Vian was sent to look at the west coast of Spitsbergen, the main island of Svalbard, which was mostly ice-free and 450 mi (720 km) from northern Norway, to assess its potential as a base. The cruisers HMS Nigeria, HMS Aurora and two destroyers departed Iceland on 27 July but Vian judged the apparent advantages of Spitsbergen as a base to be unsatisfactory. The force closed on the Norwegian coast twice and each time was discovered by Luftwaffe reconnaissance aircraft.