Final answer:
The sinking of the British passenger liner RMS Lusitania by a German U-boat in 1915 outraged many Americans and contributed to the U.S. joining World War I.
Step-by-step explanation:
The sinking of the British passenger liner that angered many Americans into demanding war was the RMS Lusitania. On May 7, 1915, off the coast of Ireland, a German U-boat torpedoed the Lusitania, which sank in about twenty minutes. The attack resulted in the loss of more than 1,200 lives, including 128 Americans. Despite being warned about traveling on British vessels by the Germans and the fact that the ship was carrying war materials, the majority of Americans saw this as an act of unprovoked aggression. The event led to a significant shift in public opinion, and when Germany resumed unrestricted submarine warfare in 1917, it prompted the U.S. to move closer to entering World War I.