The outbreak of World War II found the German submarine arm well trained but deficient in numbers. From the moment of its reestablishment, the submarine force had concentrated much of its effort on validating Kommodore Karl Dönitz’s concepts for an all-out assault on enemy trade using concentrated groups of submarines under central shore-based control to locate and destroy convoyed shipping, primarily through surfaced night attacks (wolf-pack tactics). Dönitz was promoted Konteradmiral in October 1939, but shortages of U-boats, Adolf Hitler’s initial insistence on Germany’s adherence to the Prize Regulations, and demands on the submarine force for its support of surface naval operations prevented him from exploiting the potential of the wolf-pack tactics for most of the first nine months of the conflict. On average only six boats were at sea at any one time during this period, forcing them to attack individually, although some attempts were made to mount combined attacks whenever possible.
As a result of its World War I experience after 1917, Britain was quick to begin the convoying of merchant vessels. There was some initial hesitation because of the feared detrimental effect that convoys could have on the efficient employment of shipping, but when the liner Athena was torpedoed and sunk without warning on 3 September 1939, Britain took this to indicate that Germany had commenced an unrestricted campaign of submarine warfare against merchant vessels. Regular east coast convoys between the Firth of Forth and the River Thames started on 6 September and outbound transatlantic convoys from Liverpool two days later.
The conquest of Norway and the collapse of France in June 1940 brought substantial changes to the U-boat war against trade. From French bases, German reconnaissance and long-range bomber aircraft operated far into the Atlantic, while the operational range of the U-boats sailing from Norway and French Biscay ports increased dramatically. Italy’s simultaneous entry into the war terminated all commercial traffic in the Mediterranean except for very heavily escorted operational convoys bringing supplies into Malta. It also substantially increased the number of submarines available for the Atlantic campaign against shipping, inasmuch as Italian submarines began operating from Biscay ports, effectively doubling the total Axis force at sea. This situation allowed Dönitz to introduce his wolf-pack tactic on a large scale into the Atlantic shipping campaign, just as the British faced an alarming shortage of oceanic convoy escorts because of the neutralization of the French Fleet and their decision to retain destroyers in home waters to guard against a German invasion. The results vindicated Dönitz’s belief in the effectiveness of wolf packs. In the first nine months of the war, German U-boats sank a little more than 1 million tons of shipping, whereas they and the Italians together destroyed more than 2.3 million tons between June 1940 and February 1941. However, the release of destroyers from their guard duties, the addition of new escorts, and the transfer of fifty obsolete destroyers from the U.S. Navy improved the situation. The dispersal point for westbound transatlantic convoys and the pickup point for escort groups meeting eastbound shipping gradually moved westward as the range of the escorts was increased. This pushed the main arena of Axis submarine operations more toward the mid-Atlantic zone, which reduced the time that boats could spend on station. In mid-1941 the United States imposed its so-called Neutrality Zone on the western Atlantic and began escorting British convoys in conjunction with Royal Canadian Navy escorts, operating from Argentia in Newfoundland. North Atlantic convoys now were escorted throughout by antisubmarine vessels. Nevertheless, these additions to the escort force had only a limited impact on losses, since German and Italian submarines succeeded in sinking a further 1.8 million tons in the following nine months prior to the U.S. entry into the war.
The German declaration of war on the United States on 10 December 1941 brought a major westward expansion of U-boat operations against shipping. A disastrous period followed, while the U.S. Navy struggled with the problems of finding the escorts and crews required to convoy the enormous volume of merchant traffic along the East Coast of the United States, and with the very concept of convoy itself. Axis submarines sank more than 3 million tons of Allied shipping between December 1941 and June 1942, well over 75 percent of it along the East Coast of the United States and Canada. Nevertheless, by mid-1942 an elaborate and comprehensive system of interlocking convoy routes and sailings was established for the East Coast of North America and the Caribbean.
As Dönitz became aware of the extension of convoy along the Atlantic East Coast, he shifted U-boat operations back to the mid- Atlantic. His all-out assault on the North Atlantic convoy systems inflicted heavy losses: between July 1942 and March 1943, Axis (almost entirely German) submarines destroyed more than 4.5 millions tons of Allied shipping, over 633,000 tons in March alone. Nevertheless, new Allied countermeasures became available at this crucial moment, and U-boat successes fell to 287,137 tons in April, 237,182 tons in May, and only 76,090 tons in June. Dönitz’s reaction was to deploy his U-boats in areas where Allied antisubmarine forces were weak, anticipating that this would compensate for the lack of success in the North Atlantic. Initially this plan to some extent met his expectations, since sinkings rose to 237,777 tons in July, but the success of the Allied assault on U-boats in transit to their patrol stations rendered the German accomplishment transitory; merchant ship sinkings dropped to 92,443 tons in August, never to surpass 100,000 tons per month at any subsequent time during the war.
The collapse of the U-boat offensive in mid-1943 resulted from the Allies’ concurrent deployment of a series of new countermeasures and technologies that reached maturity almost simultaneously: centrimetric radar aboard both ships and aircraft, efficient shipborne high-frequency direction finding, ahead-throwing weapons that permitted ships to fire antisubmarine bombs forward and thus retain sonar contact, very-long-range shore-based antisubmarine aircraft, escort carriers and escort support groups, and advances in decryption of German communications codes. The U-boat arm attempted to defeat these countermeasures by deploying its own new weaponry, the most important elements of which were radar warning receivers, heavy antiaircraft batteries, and acoustic torpedoes designed to hunt antisubmarines vessels. Not only did these fail to stem the tide of Allied success against the U-boats, but new convoy communications codes also defeated German cryptographers, rendering locating targets much more difficult. Then, in 1944, Allied military successes in France began to force German U-boats to make more extended passages to their patrol areas as their home ports moved farther from the Atlantic; German air bases also ceased to give aircraft quick access to British coastal waters.
During the final year of this conflict, U-boats equipped with snorkels entered service. The production of new, fast elektroboote (the radical new Type XXI submarines with high underwater speed) allowed the first examples to become operational, but their numbers were far too few to make any difference. Also, there were insufficient experienced crews available to exploit their potential. Such was the success of Allied antisubmarine measures during this period that full-scale convoying became unnecessary in some areas, and much of the focus of their escorts turned to hunting U-boats rather than directly protecting merchant shipping. The full measure of the defeat of the U-boats is indicated by the fact that more than two-thirds of the 650 German submarines lost during World War II were sunk in the last two years of the war.
Italian submarines were active in the North Atlantic and the Mediterranean during World War II. In the North Atlantic they were quite successful, sinking more than 500,000 tons of shipping during about 180 patrols out of Bordeaux between mid-June 1940 and the end of 1942. In the Mediterranean, however, their successes were few and their losses high: they sank about 200,000 tons of shipping during the course of more than 1,500 wartime patrols. That was largely because of the Italian Fleet’s tactical doctrine, which envisaged using its submarines for daylight submerged attacks rather than nighttime operations on the surface, as was the German practice.
The Imperial Japanese Navy saw submarines as an essential component of its basic strategy for a Pacific war, the yogeki zengen sakusen (ambush-attrition operation). The purpose of all Japanese auxiliary forces (originally all warships other than battleships, although aircraft carriers later were given capital rank) was to wear down an advancing U.S. Fleet as it crossed the Pacific, so that it would be reduced in numbers and strength to a point that the main, untouched Japanese Fleet could crush it in a decisive battle. Submarines, therefore, operated almost exclusively as scouts and raiders against warships of the U.S. Navy and rarely against even the fleet train, let alone merchant shipping. While some boats achieved spectacular successes—such as the I-19’s salvo of four torpedoes on 15 September 1942, which sank the carrier Wasp, damaged the destroyer O’Brien so severely that it later sank, and forced the new battleship North Carolina to proceed to Pearl Harbor for repairs—most operations were unproductive. Overall, the Japanese submarine force was responsible for sinking about twenty warships and a little more than 100 merchant vessels during World War II; it suffered devastating losses itself.
Technology played a major part in determining the effectiveness of submarines. Most navies encountered problems with their torpedoes early on, especially those submarine arms that relied on magnetic rather than contact pistols. Radar development conferred a special advantage on Allied submarines in particular, offsetting the edge in optical quality possessed by German and Japanese vessels. U.S. submarines were almost unsurpassed in their level of habitability— they were almost the only boats that featured full air-conditioning and adequate space for their crews to sleep. Sonar developed rapidly, as did countermeasures, while some navies put much effort into stealth and self-defense measures by emphasizing the use of wakeless electric torpedoes and special antiescort homing torpedoes. The course of the submarine war demonstrated that those forces that fell behind in the technological battle suffered disproportionately heavily in combat.
