Monday, June 5, had long been planned for launching D-day, the start of the campaign to liberate Nazi-held Western Europe. Yet the fine weather leading up to the greatest invasion the world would ever see was deteriorating rapidly. Would it hold long enough for the bombers, the massed armada, and the soldiers to secure beachheads in Normandy? That was the question, and it was up to Ike’s chief meteorologist, James Martin Stagg, to give him the answer.

On the night of June 4, the weather hung on a knife’s edge. The three weather bureaus advising Stagg—the US Army Air Force, the Royal Navy, and the British Met Office—each provided differing forecasts. Worse, leading meteorologists in the USAAF and Met Office argued stormily. Stagg had only one chance to get it right. Were he wrong, thousands of men would perish, secrecy about when and where the Allies would land would be lost, victory in Europe would be delayed for a year, and the Communists might well take control of the continent.