Unrestricted submarine warfare is a type of naval warfare in which submarines sink vessels such as freighters and tankers without warning, as opposed to attacks per prize rules (also known as "cruiser rules"). Prize rules call for submarines to surface and search merchantmen and place crews in "a place of safety" (for which lifeboats did not qualify, except under particular circumstances) before sinking them, unless the ship showed "persistent refusal to stop . . . or active resistance to visit or search". During the First World War, the British introduced Q-ships with concealed deck guns, and armed many merchantmen, leading the Germans to ignore the prize rules; in the most dramatic episode they sank Lusitania in 1915 in a few minutes because she was carrying war munitions. The U. S. demanded it stop, and Germany did so. Admiral Henning von Holtzendorff, chief of the Admiralty staff, argued successfully in early 1917 to resume the attacks and thus starve the British. The German high command realized the resumption of unrestricted submarine warfare meant war with the United States but calculated that American mobilization would be too slow to stop a German victory on the Western Front.