U-boat is the anglicize version of the German word U-Boot (help·info), itself an abbreviation of Unterseeboot (undersea boat), and refers to military submarines operated by Germany, particularly in World War I and World War II. Although in theory U-boats could have been useful fleet weapons against enemy naval warships, in practice they were most effectively used in an economic-warfare role, enforcing a naval blockade against enemy shipping. The primary targets of the U-boat campaigns in both wars were the merchant convoys bringing supplies from the British Empire and the United States to the Island of Great Britain. Austrian submarines of World War I were also known as "U-Boats".