Because some passengers had to be transferred from another liner, the Lusitania left America two hours late. To pare costs in the face of declining wartime travel, the ship conserved coal by using just three of its four boiler rooms, which prolonged the voyage. If neither delay had happened, the Lusitania’s course probably would not have intersected U-20’s. British officials did not tell Turner to alter the Lusitania’s route to Liverpool and did not send even a single one of the available navy vessels for protection.

Churchill had spoken of attracting shipping to Britain’s shores “in the hopes especially of embroiling the United States with Germany.” And: “We want the traffic — the more the better; and if some of it gets into trouble, better still.”

In his World War II memoirs, Churchill recalled hearing about Pearl Harbor: “I thought of a remark that [Foreign Secretary] Edward Grey had made to me more than 30 years before — that the United States is like ‘a gigantic boiler. Once the fire is lighted under it there is no limit to the power it can generate.’ . . . I went to bed and slept the sleep of the saved and thankful.”

Churchill’s thinking about the Lusitania tragedy and the economizing of violence was similar. In his World War I memoir, he wrote that America’s entry into the war in April 1917 “could have been done in May 1915. And if done then what abridgement of the slaughter . . . would have been prevented.”