Owning a fragment of history — a Gettysburg bullet, a Coolidge campaign button — is fun, so in 1968 Gregg Bemis became an owner of the RMS Lusitania. This 240-meter-long passenger liner has been beneath 91 meters of water off Ireland's south coast since a single German torpedo sank it 100 years ago on May 7, 1915. It contains the 4 million U.S.-made rifle bullets and other munitions that the ship had been carrying from neutral America to wartime Britain.

It is commonly but wrongly said that the sinking altered history's trajectory. Yet some people, including Britain's first lord of the Admiralty, Winston Churchill, hoped an attack on a ship would pull America into the war. They may have facilitated the Lusitania's calamity by not taking available measures to prevent it. Of the 1,198 who perished, 128 were Americans.

In "Dead Wake: The Last Crossing of the Lusitania," Eric Larson notes that early in 1915 warfare was evolving. On Jan. 19, two zeppelins conducted Germany's first air raid on Britain. On April 22, near Ypres, Germans sent a cloud of chlorine gas drifting toward French and Canadian lines.