In the North Sea on September 22, 1914, the German submarine U-9 sank three British cruisers, the Aboukir, the Hogue and the Cressy in just over one hour.

The one-sided battle on September 22, which claimed three British cruisers and the lives of 1,400 sailors, alerted the British to the deadly effectiveness of the submarine, which had been generally unrecognized at that time. In the first few years of World War I, German U-boats took a terrible toll on Allied shipping. By 1917, continued unrestricted U-boat attacks in American vessels traveling to Britain prompted the previously neutral United States to declare war on Germany. The infusion of American ships, troops and arms into World War I, as well as the economic support the United States supplied the Allied powers, would eventually turn the tide of the war against Germany.