In the early years of the Great War, as generals waged such visible carnage that claimed so many young lives, there was another side of the conflict, invisible not only to the public of the day but to history for more than a century. Immense copper cables traversing the Atlantic carried confidential messages destined for the American diplomatic outposts in the capitals of Europe – messages sent in the service of a noble goal: bringing the slaughter to a halt. Only a short walk from the residence of the US Ambassador in London, a nondescript building on Cork Street would house a secret team of British cryptanalysts. A year into the war, that team began working to solve US codes and soon was steadily deciphering American transmissions. As the codebreakers worked, an uneasy political truce hung over the city. Beneath the surface, tension seethed amongst government ministers and unelected military and naval officials, who possessed vastly differing views of how the war was to be won – or if it remained winnable at all.