On June 5, 1944, the largest amphibious invasion force ever assembled sat poised along the southern coast of England, ready to launch Operation Overlord—the D-Day invasion of Normandy. Except they didn't. They sat there, thousands of ships packed with 150,000 increasingly seasick soldiers, because the weather was absolutely dreadful.
General Dwight D. Eisenhower had already postponed the invasion once from June 5 to June 6, gambling everything on a weather forecast from a 43-year-old Scottish meteorologist named Group Captain James Stagg. This was rather remarkable considering that Hitler's own meteorologists were predicting continuous bad weather, which is precisely why the German high command relaxed their guard. Field Marshal Erwin Rommel even went home to Germany to celebrate his wife's birthday.
Stagg, analyzing data from weather ships in the Atlantic, spotted a brief window of marginally acceptable conditions following the storm. When he presented his forecast to Eisenhower late on June 5, the Supreme Commander faced an impossible choice: launch the invasion in questionable weather or postpone again, which would mean waiting two more weeks for the right tidal conditions—risking the entire secrecy of the operation.
Eisenhower paced, solicited opinions from his commanders, and finally said simply: "Okay, we'll go."
The decision meant that one Scottish weatherman's interpretation of incomplete meteorological data essentially determined the course of World War II. Had Stagg been wrong, the invasion force could have been destroyed. Had they waited, German intelligence might have discovered the plan. Instead, history turned on a weather report.