Unrestricted submarine warfare

The law of the sea required a warship to stop a merchant vessel, search it and see to the crew's safety before sinking it. A submarine, small and defenceless on the surface, could not do this without risk. On 4 February 1915 Germany declared the waters around the British Isles a war zone. The sinking of the Lusitania in May 1915, and later of the Arabic and the Sussex, drew sharp protests from the United States, and Germany scaled the campaign back several times.

At the turn of 1916 and 1917 the naval staff and the Supreme Army Command convinced the Kaiser that sinking about 600,000 tons a month would force Britain to make peace within six months. From 1 February 1917 U-boats attacked without warning every ship in zones around Britain, France and Italy and in the eastern Mediterranean. Shipping losses were huge, but convoys turned the tide, and the decision helped bring the United States into the war in April 1917.