The Atlantic U-boat campaign of World War I (sometimes called the "First Battle of the Atlantic", in reference to the World War II campaign of that name) was the prolonged naval conflict between German submarines and the Allied navies in Atlantic waters –the North Sea, the seas around the British Isles, and the coast of France.
Initially the U-boat campaign was directed against the warships of the British Grand Fleet. Later U-boat fleet action was extended to include action against the trade routes of the Allied powers. This campaign was highly destructive, and resulted in the loss of nearly half of Britain's initial merchant marine fleet during the course of the war.