When World War I began in August 1914, both Germany and Austria-Hungary possessed small flotillas of relatively modern submarines. Germany had 31 operational U-boats, while Austria-Hungary had only 5. As with all the world’s navies at the time, the Central Powers had no clear doctrines for the employment of their submarine forces, nor did they have any real appreciation of the directions that wartime operations would take. Neither fleet’s prewar plans long survived the reality of war, since the Royal Navy adopted a strategy of distant blockade rather than the close blockade that the German Navy had anticipated, and Italy declined to join its allies in the Triple Alliance and chose to remain neutral, upsetting Austrian expectations of the situation in the Adriatic.
Germany adopted a strategy of Kleinkrieg (little war, or small engagements), seeking to draw out elements of the Grand Fleet into disadvantageous positions, both geographically and numerically, and whittle away at British naval strength with mines and submarines. During 1914 German U-boats, demonstrating considerably greater operational capabilities than prewar exercises had suggested, scored some considerable successes, most spectacularly on September 22 when Captain Lieutenant Otto Weddigen’s U-9 torpedoed and sank the three British armored cruisers Aboukir, Cressy, and Hogue off the Dutch coast within little more than an hour. The Royal Navy quickly came to regard the menace of German submarines as a grave threat to its naval superiority.
Despite the early German successes, it was clear by early 1915 that the strategy of Kleinkrieg was not working. The British distant blockade was proving all too effective in cutting off Germany’s access to most foreign trade, while the Grand Fleet, far from allowing isolated elements to fall into German traps, was succeeding in cutting off detachments of the High Seas Fleet and inflicting serious damage upon them. The German Navy was under increased threat from an intensified British mining campaign and expansion of the terms of the naval blockade.
German submarines had not conducted any coordinated campaign against Allied merchant shipping—the transport of the British Expeditionary Force to France and its subsequent continued supply had been conducted virtually without any interference from the German fleet—but had demonstrated that U-boats could be effective in this role even under the limitations of the Prize Regulations of the Declaration of London in 1909. A growing number of officers within the German Navy as well as influential politicians and businessmen began to see a counterblockade of Britain as the solution to Germany’s dilemma—in other words, to employ submarines to attack and sink without warning all British shipping and neutral vessels trading with the United Kingdom. Berlin was well aware of the potential for serious negative reaction to such policies from neutral trading nations, especially the United States, but the German leadership decided that the gains were worth the risk and on February 4, 1915, declared the waters around Great Britain and Ireland, including the whole of the English Channel and the western portion of the North Sea, to be a war zone within which any merchant ship, British or neutral, would be destroyed without it necessarily being possible to ensure the safety of its crew or passengers.
The German Navy began this first unrestricted submarine campaign against merchant shipping with limited resources. It usually had no more than about 25 operational U-boats available, of which only about one-third were deployed on station at any one time, the remainder being either in transit or refitting. The campaign began on February 28 and, despite the small number of U-boats active, achieved considerable success. A total of 29 vessels aggregating some 89,500 gross tons were sunk in March, 33 vessels totaling only 38,600 tons were sunk in April, 53 vessels totaling 126,900 tons were sunk in May, 114 vessels totaling 115,291 tons were sunk in June, 86 vessels totaling 98,005 tons were sunk in July, 107 vessels totaling 182,772 tons were sunk in August, and 58 vessels totaling 136,048 tons were sunk in September. British antisubmarine measures in this same period accounted for 15 U-boats, but the German Navy commissioned 25 new boats.
The German announcement on February 4 had almost immediate diplomatic repercussions, especially the U.S. government note warning Germany that it would be held strictly accountable for any loss of U.S. ships or lives. Consequently, the German government compromised on its initial declaration, placing some restrictions on attacks against vessels flying neutral flags much to the chagrin of German Navy officers, who envisaged that one major effect of the unrestricted campaign would be to so terrorize neutral shippers that they would cease to trade with Great Britain. A number of attacks on Dutch, Greek, Norwegian, and Swedish vessels, including some inside areas declared safe, provoked outraged diplomatic responses from these neutral governments and led the German government to offer compensation in several instances and prohibit attacks against neutral vessels.
The major blow to the unrestricted campaign was the sinking by Captain Lieutenant Walter Schwieger’s U-20 of the large Cunard transatlantic liner Lusitania without warning off the western coast of Ireland on May 7, 1915. A total of 128 U.S. citizens were among the 1,201 passengers and crew who lost their lives, and the sinking caused a major diplomatic furor between the United States and Germany that was heightened by the torpedoing of the U.S. ship Nebraskan without warning on May 25. German chancellor Theobald von Bethmann-Hollweg, despite strong opposition from the navy, forbade attacks on large passenger liners whatever flag they flew, and his efforts succeeded in mollifying President Woodrow Wilson’s government sufficiently, although Germany still suffered from a sharp drop in the American public’s estimation.
As the sinking record shows, these greater restrictions did not substantially affect the success of the campaign against merchant shipping. Nevertheless, there remained the threat of further incidents that might force Germany to terminate the campaign. Schwieger, who had sunk the Lusitania, succeeded in provoking two such incidents by sinking the British liner Arabic without warning on August 19 and the U.S. liner Hesperian on September 4. These two events provoked a further crisis between the United States and Germany and exacerbated the concerns of the German Army General Staff about increased complications with neutral nations in light of an impending shortage of troops.
Over the protests of its senior naval officers, the German government forced through a prohibition of attacks against any liners and a withdrawal of all U-boats from operations in the western approaches to the English Channel. Admiral Henning von Holtzendorff became head of the Naval Staff, and on September 18, after deciding that the submarine campaign had failed, he terminated most U-boat operations against shipping and bound all continued action to conform with the Prize Regulations, thus ending the unrestricted campaign.
The British merchant marine lost close to 1.3 million tons of shipping from all causes through the end of September 1915. New construction totaled about 1.2 million tons, and captured enemy shipping added a further 682,000 tons. Nevertheless, losses were outstripping replacements, while the sinkings in August and September were a serious concern and an omen for the future potential of a submarine campaign.
Holtzendorff continued the restricted campaign against merchant shipping. From October 1915 to February 1916, U-boats sank 209 ships totaling 506,026 gross tons, with about 75 percent of these sinkings in the Mediterranean. The campaign sharpened after attacks without warning were permitted against armed merchant vessels, beginning on February 29. During the next two months, the U-boats sank 143 ships totaling 347,843 tons, but again an incident involving U.S. citizens precipitated a diplomatic crisis. On March 24, Senior Lieutenant Herbert Pustkuchen’s UB-29 torpedoed the French cross-channel steamer Sussex without warning off Dieppe, resulting in the loss of some 50 passengers and crew, including 25 Americans. President Wilson reacted by warning Germany that any further incident would lead to the United States severing diplomatic relations. On April 24, therefore, Holtzendorff reinstated his order requiring submarines to operate within the Prize Regulations, causing commander of the High Seas Fleet Admiral Reinhard Scheer to order all submarines to cease operation. British losses fell immediately, to 64,000 tons in May and only 37,000 tons in June. Nevertheless, British shipping losses for the first half of 1916 approached 500,000 tons, well over twice the rate of new construction.
During the next few months the High Seas Fleet boats operated primarily in support of fleet operations on the North Sea, leaving attacks on merchant shipping to the Flanders boats and U-boats in the Mediterranean. The pace of the restricted campaign accelerated in September when 172 ships totaling 231,573 tons were sunk. Between October 1916 and January 1917, a further 757 ships totaling more than 1.3 million tons were sent to the bottom in all theaters. This increase reflected the larger number of operational U-boats, which reached 103 submarines in January 1917.
Despite this advance, German Navy leaders were convinced that the restricted campaign was doomed to failure as a means to bring Britain to terms. When combined with the Allied rejection of German peace proposals and successes on the Eastern Front that released additional troops, however, a consensus emerged in the high command and the German government for renewal of the unrestricted submarine warfare campaign.