This term refers to the German submarines of World War I and World War II. It derived from the German Navy's system of naming its submarines with U- followed by a number, where the U stood for Unterseeboot (literally, "undersea boat"), the German word for submarine. The primary targets of the U-boat campaigns in both world wars were the merchant convoys bringing supplies from the United States to Europe.
In May of 1915, German U-boat U-20 sank the liner RMS Lusitania. Of the 1,195 lives lost, 123 were American civilians, including a noted theatrical producer and a member of the Vanderbilt family. This event turned American public opinion against Germany, and was a significant factor in getting the United States involved in the war on the Allied side.
During World War II, U-boat warfare was the major component of the Battle of the Atlantic, which lasted the duration of the war. Sir Winston Churchill, the United Kingdom's Prime Minister for most of the war, was quoted as saying "The only thing that really frightened me during the war was the U-Boat peril." During the early stages of the war and soon after the United States' entry into the war, the U-boats were extremely effective in destroying allied shipping. Advances in convoy tactics, sonar (called Asdic in England), depth charges, the cracking of the German Enigma code, and the range of escort aircraft turned the tide against the U-boats. In the end, the U-boat fleet suffered extremely heavy casualties, losing 199 U-boats and about half of its 10,500 submariners.
During World War II, the Kriegsmarine produced many different types of U-boats as technology evolved.