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On 17 March, German submarines sank three American merchant vessels, and the U.S. declared war on Germany in April 1917. Unrestricted submarine warfare in early 1917 was initially very successful, sinking a major part of Britain-bound shipping. Unterseeboot
U-boat, German U-boot, abbreviation of Unterseeboot, (“undersea boat”), a German submarine. The destruction of enemy shipping by German U-boats was a spectacular feature of both World Wars I and II.