At the start of 1917, the Allies, most notably Great Britain, were threatened with the full force of a devastating weapon. The Germans decided to use unrestricted submarine warfare against merchant shipping. In order to capitahze on the submarine's advantages of stealth and surprise, the Germans attacked, without warning, both Allied vessels and ships bearing the flag of neutral countries. The Germans concentrated their effort in a wide zone around the British Isles, hoping to close off shipping lanes and to shut down Britain's ability to wage war. The use of the new technology of the submarine raised new questions about the rights of neutrals and the "rules" of the sea. It also led to rethinking the nature of naval warfare in the modem age. For all the vast expenditures of 1914, the huge naval armadas and even their individual vessels only rarely saw action. To the surprise of all authorities, the naval war took a novel turn. The submarine war—as the Germans used the new weapon to strike at Allied commerce, and their opponents sought desperately to find effective countermeasures—became the centerpiece of naval operations for the final two years of the war. As U-boats hunted their prey and Allied surface forces sought to combat the Germans, the course of the entire war at sea hung in the balance.