When World War I began in 1914, the United States -- which had never interfered in a European war -- was officially neutral. The land war quickly devolved into a stalemate as the Central Powers -- consisting of Germany, Austria-Hungary, Bulgaria and the Ottoman Empire -- and the Allies -- consisting of the British Empire, France, Belgium, Italy and Russia -- dug extensive series of trenches. At sea, the British Empire's surface fleet was dominant, leading the German Empire to declare unrestricted submarine warfare around the British Isles.

German Warning

The German Embassy published notices in American newspapers warning of the unrestricted submarine warfare and specifically informing travelers that they traveled at their own risk aboard Allied ships. One such warning even appeared on the same page with an advertisement for the May 1, 1915 departure of the ocean liner RMS Lusitania from New York City, bound for Liverpool, England.

RMS Lusitania

The RMS Lusitania departed New York City with 1,959 passengers aboard, 123 of whom were Americans. The German submarine U-20, under the command of Kapitänleutnant Walther Schwieger, torpedoed the Lusitania off the coast of Ireland on May 7. Lusitania sunk in 18 minutes and 1,195 of its passengers -- including many Americans -- perished. This loss of American life caused outrage in the United States and turned public opinion against Germany. Germany halted unrestricted submarine warfare, but resumed the practice in 1917, sinking four American ships and leading the United States to declare war on Germany.