At the outset of World War I, Britain gained an advantage over Germany by controlling the waters around Europe. The British navy blockaded the flow of supplies to German ports, while Britain freely moved its own troops and supplies across the water to its ally, France. Germany determined to counteract Britain’s sea power with its submarines, or U-boats, and in early 1915 warned that no vessel around the British Isles would be safe.

On May 2, 1915, the British passenger ship Lusitania left New York, bound for Liverpool. Five days later, the German sub U-20 torpedoed the liner as it approached the coast of Ireland. The ship listed heavily and sank within 20 minutes. In total, 1,198 of the nearly 2,000 passengers and crew aboard the Lusitania perished. The fatalities included 128 Americans.

While the United States had proclaimed its neutrality when World War I began, “Remember the Lusitania” became a rallying cry for American involvement. President Woodrow Wilson condemned German submarine warfare, but the U.S. remained neutral. Germany later suspended its U-boat campaign to appease the U.S., but then restarted it, believing that its submarines could force Britain to surrender before America would be ready to fight. The U.S. finally declared war on Germany in April 1917.

PostScript: In justifying its attack, Germany claimed that the Lusitania was armed and held war supplies. Though the British government denied the charge, stating that the liner carried only a limited amount of rifle ammunition, later research indicated that the Lusitania was transporting nearly 175 tons of munitions.