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The German invasion of Norway in April and rapid occupation of the Low Countries and France in May and June had transformed the war at sea by the middle of 1940. With new bases in northern Norway and western France, the German Navy could now circumvent the British blockade and reach out in to the Atlantic. Patrols could extend much further and greatly increase their time at sea. The Royal Navy had lost many destroyers and other escort vessels in withdrawing Allied land forces from Europe, but the large merchant fleets of occupied countries like Norway and the Netherlands had come under British control.

Surface raiders continued to be a potent threat and, in the disproportionate counter-measures they forced the British to take, had an impact far greater than their actual numbers should have achieved. The pocket battleship Admiral Scheer started a successful five month cruise in October 1940 sinking 16 ships, including five in a westbound Atlantic convoy on 5 November. Brave, but ultimately suicidal, resistance from the armed merchant cruiser Jervis Bay, the only escort vessel for the 37-ship convoy, prevented much greater casualties. In February 1941, the heavy cruiser Admiral Hipper sank seven ships in an unescorted convoy. Between January and March, battle cruisers Scharnhorst and Gneisenau had great success in not only capturing or sinking 22 ships, but also completely disrupting the British convoy system in the process. However, the loss of the new 42,500 ton battleship Bismarck in May 1941, on her first operation, caused Hitler to withdraw his surface raiders to Norwegian waters for the rest of the war, where they remained as a threat to convoys supplying Russia across the Arctic Ocean.

The favourable strategic position which the fall of France gave Dönitz's U-boats created what their captains and newly-emerging 'aces' like Prien, Kretschmer, Schepke and Lemp called the first "Happy Time". Fleet strength was low, with only 28 boats operational in July, 27 in October and 21 in February 1941 (the lowest total of the war). Nevertheless, from June 1940 onwards, Allied shipping losses to submarines increased steeply to a total of 3,206,096 tons (606 ships) in the thirteen months to the beginning of July 1941. Included in this grim statistic was the Empress of Britain, sunk by U-32 on 26 October, the only large passenger liner lost during the war. Allied escort vessels, which were too few in number and with many lacking in endurance, had no answer to the lone submarine attacking on the surface at night as their asdic detection apparatus (later called sonar) only worked against targets underwater. U-boat production began to rise significantly from the spring of 1941 which enabled Dönitz to consolidate new tactics he had gradually introduced over the autumn. Once a convoy had been located by radio intercept, long-range aircraft or one of the U-boats lying in line across the shipping lanes, instructions from Dönitz's headquarters at Lorient on the French Atlantic coast brought together all the other boats in search of the prey, in what the British called "wolf packs".

In response, the British took steps to extend the range and strength of convoy escort by sea and air. They were aided by a gradual increase in the number of escort vessels available. Of particular value were the fifty US First World War vintage destroyers exchanged in September 1940 for American use of British bases in the western hemisphere. Permanent escort groups were formed to improve the co-ordination and effectiveness of men and ships in action. Greater co-operation with supporting aircraft was also achieved. In February 1941, the Admiralty moved the headquarters of Western Approaches Command from Plymouth to Liverpool, where much closer contact with, and control of, the Atlantic convoys was possible. In April, it took over operational control of Coastal Command aircraft. At a tactical level, new short-wave radar sets, suitable for both ships and aircraft, which could detect surfaced U-boats began to arrive during 1941.

By May 1941, the development of naval and air bases in Iceland, which occupied a key strategic position, and the forward deployment of the Royal Canadian Navy to Newfoundland established a system of continuous escort at sea in both directions across the north Atlantic. The West Africa and Gibraltar routes were also given the same coverage by July. Also, eighteen months of wartime conditions had given the British the experience to identify essential requirements. Imports had, therefore, fallen sharply over the winter of 1940-1. The reduction in port congestion which resulted produced faster turnaround times and a more efficent shipping organisation.

In the second half of 1941, the British and Canadian improvements bore fruit. The destruction of certain boats had already ended the dominance of the U-boat 'aces' by March 1941. In May and June, far greater triumphs were scored with the capture of enigma machines and codebooks from U-110 and two German weather ships. This aided British cryptographers who, by August 1941, had regular and rapid access to U-boat signal traffic in the Atlantic, allowing many convoys to be given precise routes away from danger. Twelve U-boats had been sunk in the first six months of the year, but a further twenty-three were destroyed in the second half of the year. In contrast, Allied merchant shipping losses to U-boats fell from 1,451,595 tons (263 ships) to 720,159 tons (169 ships). Total losses for the year in the north and south Atlantic to all types of attack were 2,555,616 tons (525 ships). Also, British shipyards launched 1.2 million tons of new shipping during the year and over seven million tons were on order in America. Dönitz's force in the Atlantic was weakened by the dispersal of U-boats to Arctic, Mediterranean and Norwegian waters. The first major wolf pack offensive had been defeated. The U-boat's impact in 1941 had been serious, but was not yet decisive.

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