On Jan. 31, 1917, the Germans unleashed their U-boats in the Atlantic during World War I.
A shortening of Unterseeboot, or undersea boat, the U-boat proved to be lethal against Allied naval warships and commercial craft. Even a single U-boat could wipe out an entire convoy, provided that the boat was able to surface and attack using its deck gun. U-boats would typically attack deep into the Atlantic, but they liked to remind their targets that they weren’t safe at any time, so some U-boats were sent to hunt right off the coast.
When World War I broke out, the United States pledged neutrality, but after the Germans enforced a naval blockade of Great Britain, several U.S. ships were damaged or even sunk in the crossfire.
Trying to prevent the United States’ involvement, Germany apologized, but by 1915, the German embassy in Washington warned Americans traveling on British or American ships through war zones that they did so at their own risk. Nonetheless, Germany maintained that it would ensure the safety of passengers on unarmed vessels before sinking the ships — yet shortly after, they sank the Lusitania, killing 1,198, and an Italian liner, killing 272 more.
As a result, the U.S. turned against Germany, who announced that they would resume unrestricted naval warfare. By spring, several more U.S. and Allied ships were attacked. The United States had had enough and prepared to go to war.