In April 1915, the British Cunard liner Lusitania prepared to leave New York harbor. While the German embassy had issued a warning to travelers to cross the Atlantic at their own risk, many gave little heed to that admonition. Only one passenger canceled his ticket. On May 7, the Lusitania was passing Ireland on its way to England when a German submarine attacked, sinking the ship with 1,198 passengers onboard, including 126 Americans. Germany insisted that the Lusitania carried munitions; the United States denied the allegations (though it would later be learned that there were cases of shells, cartridges, and small-arms ammunition onboard). Even though the ship's sinking enraged Americans, who felt the Germans had attacked a defenseless civilian vessel, the Wilson administration was determined to keep the country out of war. The United States forced Germany to modify its method of submarine warfare, but in no time at all, the Germans sunk a French steamer, causing the loss of additional American lives.

Wilson won re-election in 1916 while the war in Europe raged on. The numbers of casualties mounted: in the Battle of the Somme, 1.25 million men on both sides were killed, wounded, or captured, and the Battle of Verdun resulted in 1 million French and German casualties. A year later, Germany declared all-out submarine warfare; the United States could not remain neutral much longer.