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.
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
