On May 1, as the German government was offering apologies and reparations for having sunk the unarmed American tanker Gulflight , an advertisement appeared in New York newspapers warning Americans that they traveled on British ships through the North Atlantic at their own risk. The British ocean liner Lusitania sailed from New York for England that day.

On May 7 a German U-boat torpedoed the Lusitania off the coast of Ireland, killing nearly 1,200 people, among them 124 Americans. “It is a deed for which a Hun would blush, a Turk be ashamed, and a Barbary pirate apologize,” wrote the editor of the Nation . Great Britain denied the German accusation that the Lusitania had been carrying munitions, and on May 13 the U.S. Secretary of State, William Jennings Bryan, sent the German government a letter demanding that it disavow the attack. On June 9, following Germany’s declaration that it stood by its policy of unrestricted submarine warfare, President Woodrow Wilson sent a second, much stronger note demanding that Germany guarantee the safety of “American citizens bound on lawful errands as passengers on merchant ships of belligerent nationality.”

Bryan believed the second note invited war, and he resigned his post rather than sign it. But it was already too late to preserve the neutrality of American public opinion. “Must we,” asked the Louisville Courier-Journal , “as a people sit down like dogs and see our laws defied, our flag flouted and our protests whistled down the wind of this lordling’s majestic disdain?” Winston Churchill, in his six-volume history of World War I, would name the sinking of the Lusitania as a “direct cause of [Germany’s] ruin.”