The Austro- Hungarian SM U-21 loads a torpedo during World War I.

When war began in 1914, both Germany and Austria-Hungary had small but relatively up-to-date fleets of submarines (there were 31 operational U-boats in the German Navy and 5 in the Austro- Hungarian). Like the Allies, they had no clear doctrines concerning their use. Furthermore, the prewar plans of neither fleet long survived contact with the realities of the war. The Royal Navy adopted a strategy of distant blockade rather than the close blockade that the German Navy had anticipated, while 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, seeking to draw out elements of the Grand Fleet into disadvantageous positions, both geographically and numerically, and whittling away at British naval strength with mines and submarines. During 1914, German U-boats, demonstrating much greater operational capabilities than prewar exercises had suggested, scored considerable successes. The most spectacular was on 22 September, when Kapitänleutnant Otto Weddigen, commanding the U- 9, torpedoed and sank three British armored cruisers—the Aboukir, the Cressy, and the Hogue— within little more than an hour off the Dutch coast. The Royal Navy very quickly came to regard the menace of German submarines as the greatest single threat to its naval superiority.

Despite this success, by early 1915 it was clear 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 detached elements to fall into German traps, was succeeding in cutting off detachments of the High Seas Fleet and inflicting serious damage upon them. The threat of an intensified British mining campaign and the expansion of the terms of the blockade were tightening the screw.

German submarines had not conducted any coordinated campaign against Allied merchant shipping: the transport of the British Expeditionary Force to France and its subsequent supply had been conducted virtually without any interference from the German Fleet. But German submarines had demonstrated that U-boats could be effective in that role, even under the limitations of the Prize Regulations of the Declaration of London of 1909, which required warships either to take merchant crews and passengers on board or provide for their safety prior to sinking their ships. A growing number of officers within the navy, as well as influential politicians and businessmen, began to see a “counterblockade” of Britain as the solution to Germany’s dilemma, using submarines to attack and sink without warning all British shipping and neutral vessels trading with the United Kingdom. The government was well aware of the potential for serious repercussions from neutral nations, especially the United States, but it decided that the gains were worth the risk. Therefore, on 4 February 1915, the waters around Great Britain and Ireland, including the whole of the English Channel and the western portion of the North Sea, were declared to be a war zone within which any merchant ship, British or neutral, would be destroyed, without necessarily being possible to ensure the safety of crew or passengers.

The German Navy began this first unrestricted submarine campaign against merchant shipping with very limited resources. Typically there were 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 28 February, and, despite the small number of active U-boats, did well. In all, 29 vessels totaling some 89,500 gross tons were sunk in March; 33 ships totaling 38,600 tons in April; 53 vessels totaling 126,900 tons in May; 114 ships totaling 115,291 tons in June; 86 ships totaling 98,005 tons in July; 107 vessels totaling 182,772 tons in August; and 58 ships totaling 136,048 tons in September. British antisubmarine measures accounted for 15 U-boats, but the German Navy commissioned 25 new boats during the period.

The German announcement of 4 February had almost immediate diplomatic repercussions, especially the U.S. government’s 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. That was much to the chagrin of officers in the fleet, who had envisaged the unrestricted campaign so terrorizing 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 those neutrals and led to the German government offering compensation in several instances and prohibiting attacks against neutral vessels.

The major blow, however, to the unrestricted campaign was the sinking of the transatlantic liner Lusitania without warning off the western coast of Ireland on 7 May 1915, by Kapitänleutnant Walter Schwieger, commanding the U-20. Among the 1,201 passengers and crew who lost their lives were 128 U.S. citizens, causing 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 25 May. Chancellor Bethmann Hollweg, despite strong opposition from the fleet, forbade attacks on large passenger liners, whatever flag they flew, and his efforts succeeded in mollifying President Wilson’s government—although Germany still suffered from a sharp drop in the U.S. 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. Kapitänleutnant Schwieger, who had sunk the Lusitania, succeeded in providing two such incidents by sinking the British liner Arabic without warning on 19 August and the U.S. liner Hesperian later in the month. Those two events provoked a further crisis between the United States and Germany and exacerbated the concerns of the German Army’s general staff about increased complications with neutral nations in light of an impending shortage of troops. The government, over the protests of senior naval officers, forced 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 18 September, after deciding that the submarine campaign had failed, he terminated most U-boat operations against shipping. He also bound all continued action to conform with the Prize Regulations, thus ending the unrestricted campaign.

The British merchant marine had lost 1,294,000 tons of shipping from all causes up to the end of September 1915. New construction totaled 1,233,00 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 (about 75 percent of those sinkings were in the Mediterranean). The campaign sharpened after attacks without warning were permitted against armed merchant vessels, beginning on 29 February. 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 further diplomatic crisis. On 24 March, Oberleutnant zur See Pustkuchen, commanding the UB-29, torpedoed the French cross-Channel steamer Sussex without warning off Dieppe, killing 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 24 April, Holtzendorff reinstated his order requiring submarines to operate within the Prize Regulations, causing Admiral Scheer angrily 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 a half-million tons, well over twice the rate of new construction.

For 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 picked up in September, when 172 ships totaling 231,573 tons were sunk. Between October 1916 and January 1917 a further 757 ships totaling 1,304,290 tons were sunk in all theaters. This increase reflected the larger number of operational U-boats, which reached a total of 103 submarines in January 1917.

Despite this advance, the navy was convinced that the restricted campaign was doomed to failure in bringing Britain to terms. When combined with the Allied rejection of German peace proposals and successes on the Eastern Front that released additional troops, the consensus among the high command and in government tipped toward renewal of the unrestricted campaign. Holtzendorff, relying on calculations of available British shipping and requirements, and assuming that neutrals would be terrorized into ceasing to trade with Britain, determined that an unrestricted campaign sinking 600,000 tons of shipping per month would reduce traffic to and from the United Kingdom by close to 40 percent within five months and force a peace settlement. There was a very real risk that such a campaign would bring the United States into the war against Germany, but it was thought that U.S. shipping and troops could not participate in time to influence the outcome. On 9 January 1917, the government decided that an unrestricted campaign would begin on 1 February.

The predicted diplomatic consequence of launching the new unrestricted campaign was not long in coming. On 3 February 1917, President Wilson severed diplomatic relations with Germany. The Zimmerman Telegram—containing a proposal for a German-Mexican and even a German-Japanese alliance against the United States—surfaced and exacerbated the tension between the two nations. A series of almost inevitable sinkings of U.S. ships or vessels carrying U.S. citizens as passengers hastened matters, and on 6 April the United States declared war on Germany.

The campaign commenced on schedule, with some 120 U-boats deployed. Shipping losses rose dramatically to 520,412 tons in February; 564,497 tons in March; and 860,334 tons in April, at the cost of only nine U-boats. From that peak, losses dropped to 616,316 tons in May and 696,725 tons in June. As the British gradually expanded the scope of convoy from mid-May, losses declined still further to 555,514 tons in July; 472,372 tons in August; 353,602 tons in September; 466,542 tons in October; 302,599 tons in November; and 411,766 tons in December. Forty-three U-boats were sunk in the same six-month period, more than three times the number lost in the first six months of the year. During the first six months of 1918, U-boats sank 1,864,440 tons of shipping and a further 889,442 tons up to the Armistice on 11 November. German losses were high—more than 120 U-boats were sunk. Furthermore, as the U-boats sought out weak links—areas in which ships sailed without the benefit of convoy—the Allies extended the scope of the system until the vast majority of shipping was covered. The German unrestricted submarine campaign against shipping had failed.

The small number of Austrian submarines in operational service at the beginning of the war and their limited range effectively confined their use to the Adriatic. After France declared war on 13 August 1914, elements of its Mediterranean Fleet commenced operations there, making sweeps with heavy ships and undertaking minor landings—all in the hope of influencing Italy to join the Allies. Austrian submarines always were a threat, one that became a reality on 21 December, when the U- 12 torpedoed the French flagship, the dreadnought Jean Bart, without, however, sinking the battleship. This incident convinced the French to give up offensive operations in the Adriatic and focus instead on a distant blockade of the Straits of Otranto.

In the run-up to the entry of Italy into the war on the side of the Allies, the French moved their blockading squadron further north into the Adriatic. Linienschiffsleutnant Georg Ritter von Trapp (better known as the Baron in the Sound of Music), commanding the U-5, succeeded in torpedoing and sinking the armored cruiser Leon Gambetta, with heavy loss of life, on the night of 26 April 1915— demonstrating once again the vulnerability of surface vessels in these confined waters. After Italy entered the war, the situation remained much the same. In the face of the threat posed by submarines, heavy ships could not operate effectively in the Adriatic, and operations became largely a quasi-guerrilla war in which the Austrian submarines, reinforced by small German U-boats transferred overland, played a major role.

Germany began transferring submarines to the Mediterranean theater in May 1915, when it became clear that the limited A u strian submarine force could do little to affect operations in the area, despite their effectiveness in the Adriatic. They scored some considerable successes on arrival. Kapitänleutnant Otto Hersing, commanding the U- 21, torpedoed and sank the British predreadnought battleship Triumph at the Dardanelles on 25 May and another, the Majestic, two days later. Two smaller submarines, however, proved very much less effective, and subsequent German submarine operations during the Dardanelles campaign proved largely inconsequential.

The German Navy continued to send U-boats to the Mediterranean, sending the smaller boats overland by rail for assembly at Pola and passing the larger boats through the Straits of Gibraltar. One early arrival, the UB-15, commanded by Oberleutnant zur See von Heimburg, sank the Italian submarine Medusa in the Adriatic on 1 June 1915. Von Heimburg, now commanding the UB- 14, then sank the Italian armored cruiser Amalfi on 7 July, before sailing into the Mediterranean.

The Mediterranean theater was attractive because there were clearly defined focal points through which much traffic had to pass. It obviously was crucial for French and Italian traffic, and much British shipping sailed through the sea. The weather would permit operations during the fall and winter, when it could hamper Atlantic operations, and it also was much less probable that problems would arise with the United States, since few U.S. ships or passengers sailed through.

The German submarine campaign in the Mediterranean began in earnest in October 1915. The submarines used Austrian bases at Pola and Cattaro for their operations. During that month five large Uboats sank 63,848 tons of shipping—more than three-quarters of all merchant vessel sinkings in all theaters. More large and small boats reinforced the Mediterranean flotilla, with merchant shipping losses reaching 152,882 tons in November and 76,693 tons in December.

During 1916 the U-boat campaign in the Mediterranean continued its successes. The first quarter of the year saw losses decline as the U-boats underwent refits, contended with winter weather, and were subject to restrictions on attacking passenger liners. Nevertheless, losses were high enough to lead the Allies to strengthen their patrol systems and divert as much shipping as possible from sailing through the Mediterranean, even though that extended voyages and tied up vessel capacity. New boats arriving from Germany and existing boats returning to service pushed merchant shipping losses in the Mediterranean for the second quarter of 1916 to 192,225 tons, about half of all losses in all theaters. These successes continued into the summer and fall, as German U-boats sank 321,542 tons of shipping between July and September. Nor was there any real relief in the fall and early winter, since a further 427,999 tons of shipping went down by the end of the year. In aggregate, German U-boats sank well over a million tons of shipping during 1916, while losing only two submarines, one of which sank after running into one of its own mines.

The beginning of 1917 brought the illusion of calm, as most of the German U-boats were refitted. Losses fell to 78,541 tons in January, before rising again to 105,670 tons in February and then dropping to 61,917 tons in March. The Germans reinforced their Mediterranean flotilla, and new Austrian boats, based on the German UB-II class, joined the German submarines in operations in the central Mediterranean. Together they sank 277,948 tons of shipping in April. The Allies were forced to reappraise their system of trade protection and began introducing convoy of merchant shipping in late May. Losses fell to 180,896 tons in May and 170,473 tons in June. Even though the Germans increased their U-boat force by more than 25 percent, losses continued to decline in July and August, to 107,303 tons and 118,372 tons, respectively. An unprofitable diversion of U-boats in the fall to support operations in Syria against the Allied offensive netted a few small warships but relieved the pressure on merchant shipping. Success returned, however, as the U-boats re-entered the campaign against merchant shipping in December, at which time losses rose to 148,331 tons. Once again, Germany’s total U-boat loss for the year was two boats, while the German and Austrian submarines sank well over 1.25 million tons of shipping.

Finally in 1918, Allied efforts to protect trade bore fruit. During the first six months of the year, German and Austrian submarines sank about 600,000 tons of shipping, but their losses rose to ten boats, twice their total losses for the previous three years. The submarines’ success rate fell dramatically in the months before the Armistice: they sank less than 250,000 tons of shipping and lost an additional four boats in the process. Moreover, these diminished accomplishments came about despite the fact that almost twice as many submarines were operational as in 1917.

Between 1915 and 1917, the German Navy operated a small number of submarines in the Black Sea. The first boats arrived in May 1915 as part of Germany’s support for Turkey during the Dardanelles campaign. Most of the boats deployed were small UB- or UC-type coastal submarines; their successes were very limited, although their operations did cause the Russians to deploy their own hunting squadrons of destroyers. The few larger submarines dispatched to the Black Sea had no greater success. In all, Germany sent three large and about a dozen small submarines to the Black Sea, losing almost half of them, mostly to mines.

In summation, then, the most important campaign in which the submarines of the Central Powers engaged—their operations against merchant shipping—came close to total success in April 1917, only to fail completely to overcome the effectiveness of the convoy system. In the process, however, they permanently redefined the role of the submarine in warfare.