By early 1915, a new threat began to materialise: submarines. At first, they were used by the Germans only to attack naval vessels, something they achieved only occasionally but sometimes with spectacular success. Then the U-boats began to attack merchant vessels at times, although almost always in accordance with the old Cruiser Rules. Desperate to gain an advantage on the Atlantic, the German government decided to step up their submarine campaign, as a result of the British declaring the North Sea a war zone in November 1914. On 4 February 1915, Germany declared the seas around the British Isles a war zone: from 18 February Allied ships in the area would be sunk without warning. This was not wholly unrestricted submarine warfare as efforts would be taken to avoid sinking neutral ships.