On the morning that the British-owned passenger liner the Lusitania was set to sail from New York harbor, readers of several New York dailies found startling advertisements in their newspapers. The Imperial Germany Embassy had taken out full-page notices warning travelers that they boarded the Lusitania at their own risk.

In early 1915 the German navy had declared that all ships entering British territorial waters would be fired upon. Even unarmed passenger ships sailing the North Atlantic were at risk from German submarines, or U-boats. President Wilson found this an intolerable situation and vigorously defended freedom of the seas for neutral nations.

Several days after the Lusitania left New York, when the ship was just twenty miles off the British coast, a torpedo struck it and it sank like a stone, thanks in part to its illicit cargo of weapons. Of the...