<P> The initial U.S. response was to threaten to sever diplomatic ties, which persuaded the Germans to issue the Sussex pledge that reimposed restrictions on U-boat activity . The U.S. reiterated its objections to German submarine warfare whenever U.S. civilians died as a result of German attacks, which prompted the Germans to fully reapply prize rules . This, however, removed the effectiveness of the U-boat fleet, and the Germans consequently sought a decisive surface action, a strategy that culminated in the Battle of Jutland . </P> <P> Although the Germans claimed victory at Jutland, the British Grand Fleet remained in control at sea . It was necessary to return to effective anticommerce warfare by U-boats . Vice-Admiral Reinhard Scheer, Commander in Chief of the High Seas Fleet, pressed for all - out U-boat war, convinced that a high rate of shipping losses would force Britain to seek an early peace before the United States could react effectively . </P> <P> The renewed German campaign was effective, sinking 1.4 million tons of shipping between October 1916 and January 1917 . Despite this, the political situation demanded even greater pressure, and on 31 January 1917, Germany announced that its U-boats would engage in unrestricted submarine warfare beginning 1 February . On 17 March, German submarines sank three American merchant vessels, and the U.S. declared war on Germany in April 1917 . </P> <P> Unrestricted submarine warfare in early 1917 was initially very successful, sinking a major part of Britain - bound shipping . With the introduction of escorted convoys, shipping losses declined and in the end the German strategy failed to destroy sufficient Allied shipping . An armistice became effective on 11 November 1918 and all surviving German submarines were surrendered . Of the 360 submarines that had been built, 178 were lost, but more than 11 million tons of shipping had been destroyed . Of the 178 submarines destroyed, SM U-103 was sunk when the troopship RMS Olympic rammed it as it attempted to crash - dive, killing all on board . </P>

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