To understand this result, a better understanding of the submarine campaigns waged by both Germany and the United States can be achieved by examining how the war was conducted at the operational level. In minimizing a recounting of convoys attacked, tonnages sunk and submarines lost, however, this article will suggest the way German and American naval leaders planned to fight at the operational level fundamentally shaped the course of the Second World War at sea. As such it will relate how respective ideas of naval warfare came to be practiced – and then modified – by the ultimate arbiter of military success, combat.
Before the Second World War two competing interpretations of submarine doctrine vied for authority among those planning Germany’s U-boat actions. These visions foresaw submarines employed in a fleet support role, or else as an economic warfare arm of their own. The first of these saw the U-boat as an adjunct to the German surface ship fleet, one operating as a scout for the service’s heavier warships, or as a way to erode an opposing battlefleet’s strength. Such a use largely mirrored the Imperial German Navy’s use of U-boats during the battle of Jutland in 1916.3 While integral to the German operation, U-boats failed in that engagement, proving unable to attack the relatively fast moving British Royal Navy warships-specifically the battle cruisers of British Admiral David Beatty-successfully. But several elements of the deployment at Jutland played themselves out in the U-boat doctrine of the German Navy from that time onward.
After the First World War, German naval doctrine continued to consider using submarines to support surface ship operations. Against the superior British Royal Navy, submarines could provide an advantage by either detecting the enemy early or else by acting as a sort of mobile minefield to wear away the superior numbers of enemy surface warships. In the interwar exercises of the German Navy, and as late as its 1940 operations off Norway against Allied warships and convoys bound for the USSR in 1942, U-boats and surface warships tried to cooperate as teams to enhance the impact of both the surface and subsurface warships.4 German doctrine admitted a mutually supporting function, for both submarines and surface warships, to magnify the power of both against enemy combatants and merchant vessels.
An example of the fleet support doctrine to combine surface and submarine forces arose in the German Navy’s 1938 winter wargame, which posited a war against France and the USSR in 1940. In the exercise, U-boats were deployed in the Baltic Sea to serve as a reconnaissance force and act as a barrier against the Soviet Navy. In particular, multiple submarines established a picket line in the western Baltic to cover the western end of the German coast, others covered the entrance to the Gulf of Finland, and still another group formed a similar picket line in the North Sea and English Channel against the French Navy. These were anti-warship operations, designed to block any attacks on Germany or its merchant shipping in the Baltic Sea. Thus the pre-Second World War German Navy projected a need to establish U-boat reconnaissance lines to search for enemy surface warships in case of a conflict. In contrast, during the 1938 exercise, only one U-boat sailed to the Atlantic, along with several Panzerschiffe or “pocket” battleships, to attack commercial French shipping.5
The second thread in inter-war German submarine doctrine was economic warfare. The main proponent of the competing U-boat doctrine employed in the Second World War would be Karl Dönitz, from 1935 the senior submarine leader in the German Navy. Dönitz set out a limited sketch of these theories in a book first published in 1938, Die U-Bootswaffe, and distributed in two more editions by 1940.6 Dönitz planned to employ his part of the German Navy for the actions he called “cruiser warfare.” Plainly put, Dönitz specifically sought to target enemy merchant shipping in convoys, although in his inter-war writing, he always considered such tactics in the light of the so-called “Prize Rules” which mandated the stopping and searching of merchant ships.
Several tactical considerations figured prominently in Karl Dönitz’s doctrinal calculations. Submerged invisibility constituted the prime advantage of the submarine as a weapon, argued Dönitz, but one obtained at the cost of certain liabilities as well. In particular submerged submarines would be slower and therefore less likely to gain a successful firing position on their targets. Submerged submarines also suffered from a limited view through their periscopes, and would stand a better chance of detecting targets by using observers on the boats’ conning towers. Dönitz’s solution to these drawbacks lay in operating U-boats on the surface as much as possible. Additionally, Dönitz’s doctrine was specifically crafted to take advantage of several technical developments in German submarines and weapons, such as wakeless torpedoes and a better command and control system.
Two problems remained for the U-boat force. First, submarines possessed only a limited scouting capability due to limited visibility from the conning tower of a surfaced submarine – about 20 kilometers unless smoke from a potential target could be seen. To ameliorate that problem, Dönitz resolved to employ U-boats in picket lines, Vorpostenstreifen, just as the submariners had practiced in combating surface warships.7 U-boats in these reconnaissance lines would sail along parallel courses in the Atlantic, roughly 22 kilometers apart, in the hope that one of their number would spot enemy shipping in the sweep.
The second problem faced by Dönitz proved tactical and more fundamental, but its solution also drew upon the Imperial German Navy’s First World War convoy experience. In that conflict the Allied response to U-boats seeking to attack merchant shipping, grouped into a convoy, presented submarines with a difficult challenge. Not only did the U-boats face the prospect of fewer, if bigger, targets, but Allied convoys would be fairly well defended. For Dönitz, the solution to overcoming convoy defenses was simple: overwhelm them with more attacking U-boats. Dönitz labeled these methods “Gruppentaktik,” group tactics, and since then they have commonly been referred to as “Rudeltaktik,” or “Wolf Packs.”8 A U-boat on a picket line, after spotting a convoy, would refrain from attacking, and instead turn to follow, maintain contact, and broadcast a locator signal to allow the other submarines in its patrol line to close and attack. This much is generally well known.
But one other element needs mentioning – the German response to the threat posed by improved anti-submarine detection devices. Technical improvements in passive sonar allowed identification of submerged submarines, traveling at moderate speeds, at distances of up to 700 meters, while slower-moving U-boats were undetectable.9 Surfaced submarines operating with diesel engines could be heard much further away, at up to 4,000 meters. Active sonar sets could detect a submerged submarine at up to 8,000 meters, if the searching vessel were steaming slowly. Most importantly, however, a U-boat on the surface could only be spotted by active sonar at a range of just 1,000 meters and stood less of a chance of detection than did a submerged one. A skillful U-boat approach on a convoy, attacking on the surface at short range, especially at night, offered the potential for impressive success against a superior opponent. While risky, such tactics took advantage of the adequate speed, low silhouette, and relatively small wake of surfaced U-boats to sneak into the midst of a convoy, achieve surprise, and escape in the ensuing confusion.
At least initially, both of these competing doctrines were exercised successfully during the Second World War, and U-boats played a supporting role for the invasion of Norway in April 1940. The Germans’ coastal deployment patterns duplicated those of the 1916 Jutland operation, dividing their attention to locations up and down the coast and striving to attack Allied warships. For Norway, almost every U-boat in the German Navy, including training craft, participated in the operation, and six vessels were lost.10 Thus, U-boats supported the German Navy’s most successful large-scale undertaking of the war in the face of Allied, especially British, superiority.
But the grievous German surface vessel losses in the Norway operation essentially eliminated the chances for their further cooperation with submarines. Virtually nothing came of the German Navy’s further attempts to coordinate attacks on merchant shipping by U-boats and surface vessels. German submarines and surface ships only twice succeeded in cooperating to attack British merchant shipping in February and March 1941. In May 1941 the U-boat force deployed eight submarines, half its strength, to support the Bismarck and heavy cruiser Prinz Eugen, but failed to save the battleship and achieved no success against Allied shipping.11 The lack of German surface warship operations precluded further submarine-surface warship cooperation, except on a small scale off Norway against Murmansk-convoys. Thereafter, German submarines would be on their own in fighting Allied shipping.
As the Battle of the Atlantic transpired over the course of the Second World War, German group-tactics at first managed to score some significant successes. Using the group tactics in October 1940 operations against the convoys SC 7 and HX 79, in the Bay of Biscay, nine U-boats sank 33 merchant ships of 155,000 tons. This was the period when the German doctrine of commerce war proved ascendant. But the Achilles heel of the force, of course, was its over-reliance upon radio communications. Over time, these would be exploited by the Allies, who employed signals intelligence, code-breaking, and High Frequency Direction-Finding, or HF/DF, to thwart the avoid U-boat campaign. Moreover, the Allies strengthened convoy escorts by including small aircraft carriers and land-based airplanes, and together, these measures could fend off attacks by a dozen or more U-boats. The grim spring and summer of 1943 saw the end of the U-boats’ group tactics. In the new, more hostile environment, the force’s only resurgence, the so-called “inshore” offensive of late 1944 and 1945, proved the value of employing single submarines close to shore and not in the mid-Atlantic.12
For Kurt Aßmann, the “transformation” of naval warfare suggested by the seeming success of the German U-boat experience up to October 1942, would be ephemeral. Bereft of surface ships with which to exercise combined operations and employing an outmoded U-boat doctrine in the face of Allied material, technical, intelligence, German U-boats not only failed to strangle Allied shipping and win the Second World War, but suffered enormous losses in ships and personnel.
American submarine doctrine differed fundamentally from German thinking, most notably in its initial refusal to consider merchant shipping as legitimate targets for U.S. Navy submersibles. In large measure, the American attitude stemmed from an assumption that the Navy’s opponents would be nearly equal in strength to our own. In such a conflict, the surface fleet was expected to win a large-scale battle, then employ an economic blockade on the lines of Britain’s during the First World War. Under this vision, American submarines would fight other warships and – under the strictures of the 1930 London Naval Conference – attack merchant vessels only in ways that obviated the submarine’s advantage of concealment.