In April 1917, German U-Boats were sinking a dozen British ships every day. The First Sea Lord John Jellicoe,warned that Britain might not be able to carry on fighting into 1918. In the second of this two-part series, David Hayman explores Germany's World War I U-Boat threat, and the fascinating and dastardly ploys Britain used against the submarines.

Originally released by BBC, 2015. Streaming video file encoded with permission for digital streaming by Infobase on May 22, 2019.

Submarine Warfare (2:16); --Cargo Ship (4:23); -- U-Boat Fleet (3:21); -- Lusitania (4:20); -- Submarine Defenses (5:36); -- HMS Hampshire (6:23); -- British Airships (6:38); -- Unrestricted U-Boat Attacks (7:18); -- North Sea Mine Barrage (7:27); -- Final U-Boat Attack (6:02); -- End of German Fleet (4:40); -- Credits: The War at Sea: The Battle of the U-Boats (0:29);.