On June 4, 1944, with 150,000 Allied troops already loaded onto ships, one stubborn Irish meteorologist named James Stagg told Eisenhower the weather was wrong — and everyone else in the room thought he was crazy. What followed was a 36-hour argument in a Scottish manor house that determined whether the most important military operation in human history would succeed or become a catastrophe. You know how D-Day ends, but you have no idea how close it came to never starting.