Map showing ships sunk by German U-boats from February 1917 to February 1918 during World War I (1914-1918). In 1917 the German Navy began unrestricted submarine warfare aimed at all shipping travelling to the British Isles in an effort to end the war. The early success of this campaign brought the USA into the war on 6th April. The deployment of warship convoys for transatlantic crossings reduced the shipping losses and also increased the number of U-boats destroyed.