by December 1915. Arming of merchant ships pushed Germany away from prize rules towards unrestricted submarine warfare, an important factor in bringing the United States into the war against Germany. The first merchant ship lost to U-boats was an 866-ton British steamer outbound from Grangemouth to Stavanger with a cargo of coal, iron plate, and oil. was stopped by the German submarine U-17 on 20 October 1914; and a boarding party gave Glitras crew time to disembark into lifeboats before sinking the ship by opening valves to flood holds with seawater. The procedure followed customs originated by surface ships. International maritime law required the naval vessel to make adequate provisions for the safety of the merchant crew and passengers before sinking their ship. The German Empire focused use of U-boats against merchant shipping in response to British blockade of German merchant shipping by declaring the entire North Sea a war zone on 2 November 1914. On 5 February 1915 Germany published notice declaring a war zone in all waters around the United Kingdom of Great Britain and Ireland. Within that zone, Germany conducted unrestricted submarine warfare against merchant ships from 18 February 1915 without warning and without regard to safety of their crew. U-boats still conformed to earlier conventions of stopping ships when possible, but the typical submarine mounted only a single gun. The two procedures for sinking merchant ships were compared in 1915. Merchant ships escaped 42% of torpedo attacks made without warning, in comparison to 54% escaping from conventional surface attempts to stop the ship. Guns aided escape and about one submarine per month was sunk (most by Q-ships) while attempting a surfaced stop. The number of civilian merchant ships armed with anti-submarine guns rose to 1,749 by September 1916 and 2,899 by February 1917. The United States responded to unrestricted submarine warfare by severing diplomatic relations with Germany on 3 February 1917. A filibuster in the United States Senate temporarily delayed President Woodrow Wilson's proposal on 26 February 1917 to arm United States merchant ships, but arming started in March under an executive order. World War II Old naval guns had been stored since 1918 in ports for possible use. In the Second World War the objective was to equip each ship with a low-angle gun mounted aft as defence against surfaced submarines and a high-angle gun and rifle-calibre machine guns for defence against air attack. 3,400