U-boat

Germany began the war with few submarines, but on 22 September 1914 U-9 sank three British cruisers within an hour. In February 1915 Germany declared the waters around the British Isles a war zone; the sinking of the Lusitania in May 1915 and further incidents costing American lives forced it to restrict its attacks. On 1 February 1917 it resumed unrestricted submarine warfare, hoping to starve Britain into peace before America could help. Shipping losses were enormous until convoys were introduced from mid-1917.

During the war Germany built more than 370 U-boats, including the coastal UB type and the UC minelayers. They sank several thousand ships, and nearly half of the boats were themselves destroyed. After the Armistice the survivors were handed over to the Allies. Other navies also had submarines, among them the Austro-Hungarian in the Adriatic and the British in the Baltic and the Dardanelles.