On May 7, 1915, the RMS Lusitania was torpedoed off the south coast of Ireland by a German U-boat. The luxury liner, once the largest passenger ship in the world, was making one of its regular transatlantic cruises from New York to Liverpool. Most of the passengers and crew were killed when the ship went down in 18 minutes: 1,998 lost their lives, including 128 Americans.

At the time, the U.S. was neutral in what was still a European war. But no other single event of what became World War I would have “a more jolting effect on American public opinion,” said historian Arthur Link. It was a key factor in the U.S.’s alliance with the Allied Powers of Great Britain and France and set the stage for the U.S.’s declaration of war against the German Empire two years later on April 6, 1917.

Frank Trommler shows how the attack was a disaster for German-American relations and for the large numbers of Americans of German decent. The “Lusitania Effect” as he calls it, “unleashed an increasingly emotional drive of exclusion in the name of forging a new unity of the American nation. In the broader context the persecution of German Americans reinforced hysteria against socialists and other dissenters for the next half century.”

Conspiracy theories were very much a part of this. How many munitions were the ship actually carrying? Why was there a second explosion after the torpedo hit? Had Winston Churchill, then the UK’s First Lord of the Admiralty, used the ship as bait to provoke an outrage that would force Americans into the war?

Once the ship was sunk, the fact that Germany considered it to be carrying contraband of war became irrelevant. Americans were shocked and outraged, as they had been at the sinking of the USS Maine in Havana Harbor in 1898 (which probably had nothing to do with a Spanish attack) and then, 26 years after the Lusitania, the attack on Pearl Harbor. The destruction of the World Trade Center in 2001 roused the same pattern of hunting for the enemy within, and, notably, similar conspiracy theories.