The Battle of the Atlantic

Britain depended on imported food, fuel and raw materials. After 1941 the same routes carried American troops, vehicles and aircraft to Britain, where they were assembled for the invasion of Europe. A northern route carried supplies to the Soviet Union. Dönitz, who commanded the U-boat arm, wanted to sink Allied ships faster than the Allies could build them. Churchill wrote after the war that the U-boat peril was the only thing that ever really frightened him. Historians note that he gave the daily conduct of the campaign less attention than his memoirs suggest.

The Admiralty revived the convoy system within days of the declaration of war, as in 1917–18. Ships in convoy were sunk at a far lower rate than ships sailing alone. Dönitz answered convoy with the wolfpack. His boats formed patrol lines across the expected routes. The first boat to sight a convoy shadowed it and reported by radio, and the others closed to attack together, usually on the surface at night. This worked well in 1940 and early 1941. Escort radar was scarce, and asdic was designed to find submarines beneath the surface, so a boat running on the surface in the dark was hard to detect.

The fall of France in June 1940 gave the U-boats bases at Lorient, Saint-Nazaire and Brest. Boats no longer had to sail around the British Isles, and they spent more of each patrol in the convoy lanes. In December 1941 the United States entered the war. Dönitz sent his boats to the American coast, where merchant ships still sailed alone. The first attack came on 12 January 1942. Coastal convoys were organised from May, and the offensive moved to the Gulf of Mexico and the Caribbean.

Both sides read each other's signals. Bletchley Park was reading important U-boat traffic in 1941. On 1 February 1942 the U-boats introduced a four-rotor Enigma, called Shark by the British, and Bletchley lost the traffic until 13 December. Documents recovered from U-559 in October helped break it again. The German B-Dienst read the British convoy-routing cypher from 1941 and went on reading it for most of 1943. The Admiralty changed the cypher in June 1943, after the U.S. Navy's cryptologists urged it.

The crisis came in March 1943. Convoys HX 229 and SC 122 were routed on converging courses, and between 16 and 19 March they met three wolfpacks in the Mid-Atlantic Gap. The packs sank 22 merchant ships in the largest convoy battle of the war. The Allies lost 120 ships that month, 82 of them in the Atlantic. Roskill, the Royal Navy's official historian, judged that the Germans never came closer to cutting Atlantic communications than in the first twenty days of March.

The balance reversed within eight weeks. Very-long-range Liberators and escort carriers now covered most of the air gap, and escorts and aircraft carried centimetric radar and HF/DF. Support groups could leave a convoy to hunt the boats that had found it. Between 29 April and 6 May, convoy ONS 5 lost 13 ships, but its escorts and aircraft sank six U-boats. Two weeks later SC 130 crossed without losing a ship, and three U-boats were sunk. Of the 56 U-boats sunk in April and May, 36 fell to ships and aircraft working with or in support of convoys. Allied shipyards, above all American ones, were by then launching more merchant ships than the U-boats could sink. Dönitz withdrew the packs from the North Atlantic on 24 May.

Dönitz sent the packs back in September 1943. Twenty-one boats carried the new Zaunkönig acoustic torpedo, and they had orders to attack the escorts first. Against convoys ONS 18 and ON 202, between 19 and 23 September, they sank three escorts and six merchant ships but lost at least two U-boats. Within weeks the Allies fitted the Foxer noisemaker to their escorts. In October six U-boats were destroyed around ONS 20 and ON 206, for the loss of one merchant ship.

By September 1944 the Allied armies had driven the U-boats out of their French bases. Boats fitted with the snorkel moved into British coastal waters. In shallow water they could lie on the bottom and send few radio signals, which reduced the value of direction finding and decryption. The inshore campaign lasted to the end of the war. On the night of 7 May 1945 U-2336 torpedoed the freighters Avondale Park and Sneland I in the Firth of Forth. They were the last merchant ships sunk in the campaign. Dönitz had ordered the U-boats to cease attacks three days earlier.

Stephen Roskill's official history, The War at Sea, supplies the British operational narrative. Admiralty war diaries and convoy files in The National Archives supply the detail. The B.d.U. war diary records Dönitz's orders to the boats, and it survives with other U-boat command records in the Bundesarchiv. For the American coast and the Arctic route the sources are Samuel Eliot Morison's volumes, the U.S. Navy's History of Convoy and Routing and the Naval History and Heritage Command's studies of Drumbeat and PQ 17.

Signals intelligence has to be read on both sides. Bletchley Park broke important naval Enigma traffic in 1941. It was without Shark from 1 February to 13 December 1942. It lost Shark again from 10 to 19 March 1943, then broke it on 90 of the 112 days to 30 June. The German B-Dienst read British convoy-routing signals until the Admiralty changed the cypher in June 1943. Accounts differ on how much decryption contributed in May. The Naval History and Heritage Command ranks Allied intelligence against the U-boats at its weakest point of the war that month, and credits air power, radar, escort groups and production. The decrypt record shows Shark being read on most days. Loss figures follow the Naval History and Heritage Command and are rounded, because sources vary with what they count.