“The only thing that ever really frightened me during the war was the U-boat peril,” British Prime Minister Winston Churchill
An U-boat was a type of military submarine used by Germany. They were most active in the First Battle of the Atlantic in World War I and in World War II. Although they could be very effective as weapons against enemy warships, they were most effectively used in economic warfare (commerce raiding) and enforcing naval blockades against enemy shipping.
The name is an abbreviation of the German Unterseeboot (“undersea boat”). In English, U-boat refers exclusively to the German vessels used during the World Wars. In German, however, the term U-Boot refers to any kind of submarine.