In the North Sea, the German U-9 submarine sinks three British cruisers, the Aboukir, the Hogue, and the Cressy, in just over one hour. The one-sided battle, during which 1,400 British sailors lost their lives, alerted the British to the deadly effectiveness of the submarine, which had been generally unrecognized up to that time.
The German U-boat was a submarine far more sophisticated than those built by other nations at the time. The typical U-boat was 214 feet long, carried 35 men and 12 torpedoes, and could travel underwater for two hours at a time. In the first few years of World War I, the U-boats took a terrible toll on Allied shipping. Germany's quarantine of the British Isles was almost successful, but in 1917 unrestricted U-boat attacks on neutral American vessels traveling to Britain prompted the U.S. entrance into the war. The infusion of American ships, troops, and arms into World War I turned the tide of the war against Germany.