Nothing can stop an enemy from picking wireless messages out of the free air - and nothing did. In England, Room 40 was born . . .

In January 1917, with the First World War locked in terrible stalemate and America still neutral, German Foreign Secretary Arthur Zimmerman gambled the future of the conflict on a single telegram. But this message was intercepted and decoded in Whitehall's legendary Room 40 - and Zimmerman's audacious scheme for world domination was exposed, bringing America into the war and changing the course of history. The story of how this happened and the incalculable consequences are thrillingly told in Barbara Tuchman's brilliant exploration.