In most navies prior to World War II, the principal missions of submarines were offensive operations against enemy warships and reconnaissance. Britain, France, The Netherlands, and the United States, in particular, deployed many of their long-range boats in the Far East as counters to the powerful Imperial Japanese Navy concentrated there. A secondary task was campaigning against enemy merchant shipping, within the limits permitted by the Prize Regulations of the 1909 Declaration of London, which required ensuring the safety of ships’ crews.
For the first year of the war, Allied submarines operated predominantly in their primary prewar roles, undertaking patrols in the North Sea, Atlantic, and Mediterranean to find and attack enemy warships, with generally limited success. They also undertook a campaign against the German merchant fleet’s attempt to return to its homeland, with mixed results. In September 1940, however, the Admiralty lifted almost all restrictions on submarine operations against enemy merchant shipping, setting the stage for a sustained campaign against German and Italian trade.
Norwegian and Swedish ore traffic formed Germany’s most important European trade, and their protection became the principal focus of the navy’s trade protection efforts throughout the war. After the successful German invasion of Norway, the Kriegsmarine began convoying merchant shipping along the Norwegian coast from late in 1940. Convoys generally were small—three to six ships—escorted by a few torpedo boats, trawlers, and light craft. British submarines maintained a continuous effort against this trade for the remainder of the war, both by attacking convoys with torpedoes and by laying extensive minefields along the various routes they followed. After the German assault on the Soviet Union, Soviet Northern Fleet submarines initiated attacks on German shipping in northern Norway, and they were soon joined by British submarines operating from Kola Bay. Joint operations continued until 1944, when the British crews were brought home and the submarines were passed to the Soviet Navy. This assault against the northern Norwegian convoys cost the Germans some 500,000 tons of shipping, a relatively negligible amount, considering that annual traffic was well in excess of 6 million tons.
Soviet submarines also threatened the Swedish ore traffic. This shipping was encouraged to keep within Swedish territorial waters as far as possible, and was escorted for the final leg behind the protection of defensive minefields and net barriers. During 1942 and 1943, Soviet submarines succeeded in sinking only about twenty ships, totaling some 40,000 tons, out of more than 1,900 vessels in convoy representing well over 5.6 million tons of shipping. During 1944 the Soviet army’s advances and the defeat of Finland meant that aircraft played a greater role in antishipping operations, but, even so, German losses remained relatively light. The collapse of German positions on the Baltic coast early in 1945 required the evacuation by sea of more than 2 million troops and refugees. Despite some spectacular successes—the sinking of the liners Wilhelm Gustloff (with some 9,300 casualties), General Steuben (when only 300 survived of the 4,000 passengers embarked), and Goya (with but 183 survivors from among the more than 7,000 passengers and crew)—Soviet attacks were remarkably ineffective, the Germans losing only about thirty ships totaling some 100,000 tons. Overall, the Allied submarine campaign against German shipping was both relatively ineffective and costly (sixteen Allied and more than forty Soviet submarines were lost): it faced strong and effective countermeasures, especially very efficient radio direction finding and relatively powerful convoy escorts, and it was conducted in confined coastal waters that eased the defender’s task.
When Italy entered the war in June 1940, British submarines, in conjunction with surface warships and aircraft, commenced an interdiction campaign against convoy traffic carrying supplies to Italian forces in Libya almost immediately. Italian convoys generally were small—three to six merchant vessels—with two or three escorting destroyers or torpedo boats. As British surface forces operating from Malta began attacking Libya-bound shipping, the Italian Navy had to deploy heavier covering forces, often including cruisers, to support particularly valuable convoys. In this struggle over shipping, the British possessed two great advantages: radar, which vastly enhanced the night-attack capabilities of its aircraft and surface ships; and signals intelligence, which consistently gave them advance convoy routing information. Axis fortunes in this campaign fluctuated greatly. From mid-1941, Axis forces in North Africa required approximately 100,000 tons of supplies each month. In March 1942, for example, only 47,588 tons got through, whereas in April, 150,389 tons arrived. Overall, the Italian and German navies succeeded in bringing about 80 percent of all convoyed shipping in the Mediterranean through to its destination, losing two cruisers and seven destroyers to submarines in the process, while Axis antisubmarine forces and mines destroyed more than forty-five Allied submarines during the course of the campaign.
By far the most successful Allied submarine campaign during World War II was that of the U.S. Navy’s boats in the Pacific against Japanese merchant shipping and warships. The effort got off to a mediocre start, not least because the early loss of the Philippines forced U.S. submarines to traverse long distances from either Hawai’ian or Australian bases to reach their targets. In addition, Australian bases initially lacked adequate facilities, U.S. torpedoes were highly unreliable and required major attention to correct their deficiencies, and fleet doctrine required submarines to prioritize their efforts against Japan’s major warships. It took time to fix these problems: bases in Australia were not adequately equipped until the fall of 1942; it was June 1943 before destroying Japan’s merchant fleet became a doctrinal priority; and the torpedo problem was not corrected until October of 1943. During this time, however, the Pacific Fleet was able to commission modern boats, upgrade existing vessels, and substantially improve the quality of its submarine commanders.
Japan began the Pacific War with 6,150,000 tons of merchant shipping. New construction and captures added 832,000 tons during 1942. U.S. (and small numbers of British and Dutch) submarines sank 620,600 tons of Japan’s total merchant shipping losses of 1,065,000 tons during that year. During 1943, Japan added 878,000 tons of new merchant shipping to its fleet, but U.S. submarines alone sank 1,340,000 tons; another 441,000 tons were lost to other factors (air and surface attack and maritime hazards).
Despite its direct experience of successful convoy operations by its destroyers in the Mediterranean during World War I, the Imperial Japanese Navy was very slow to introduce convoying of merchant shipping after the Pacific War began. The navy possessed very few suitable escort vessels at the outbreak of the war, but that simply reflected the overwhelming emphasis it placed on planning for the decisive fleet action that was the centerpiece of its operational strategy. Japan’s response to the burgeoning unrestricted submarine campaign by the United States against its shipping was to increase aggressive surface and air patrols and to continue to eschew defensive convoy of its traffic. Not until the end of 1943, when its merchant fleet was being devastated by U.S. submarines, did the navy begin convoying shipping on a limited scale—especially the crucial tankers bringing fuel from the Dutch East Indies.
The Pacific Fleet responded by introducing the wolf-pack tactics that had served German U-boats so well in the Atlantic. In particular, coordinated night attacks by surfaced submarines relying on guidance from sophisticated radar proved devastating. As a result, 1944 was a catastrophic year for Japan’s merchant fleet. Even though Japanese shipbuilders launched 1,735,000 tons of new merchant shipping, U.S. submarines sank 2,430,000 tons, and Japan lost a further 1,550,000 tons to other causes. Shortage of targets and a highly successful aerial mining campaign in Japanese home waters cut the successes of U.S. submarines during 1945 to 400,000 tons of shipping, but the net outcome of the campaign was to all but paralyze Japan’s maritime transportation system.
Overall, U.S. submarines accounted for two-thirds of all of Japan’s losses of merchant ships, some 4.8 million tons, as well as one-third of its warship casualties. The price of this success was high. Despite the generally low level of effectiveness of Japanese antisubmarine measures, fifty submarines and 3,500 crewmen were lost during the campaign, a little more than one-fifth of all that undertook operational missions.
