On May 1, 1915, with WWI entering its tenth month, a luxury ocean liner as richly appointed as an English country house sailed out of New York, bound for Liverpool, carrying a record number of children and infants. The passengers were surprisingly at ease, even though Germany had declared the seas around Britain to be a war zone. For months, German U-boats had brought terror to the North Atlantic. But the Lusitania was one of the era’s great transatlantic “Greyhounds”—the fastest liner then in service—and her captain, William Thomas Turner, placed tremendous faith in the gentlemanly strictures of warfare that for a century had kept civilian ships safe from attack. Germany, however, was determined to change the rules of the game, and Walther Schwieger, the captain of Unterseeboot-20, was happy to oblige. Meanwhile, an ultra-secret British intelligence unit tracked Schwieger’s U-boat, but told no one. As U-20 and the Lusitania made their way toward Liverpool, an array of forces both grand and achingly small—hubris, a chance fog, a closely guarded secret, and more—all converged to produce one of the great disasters of history.