One bad decision could have altered history.
Allies were ready to storm the beaches of France occupied by Nazi Germany. American forecasters were giving the green light for Operation Overlord to commence on June 5th. However, the British Meteorological Office warned that storms were on the way and would affect operations.
Captain James Stagg, a British Officer and Meteorologist, had a final say in whether the Allies would invade or not. He made the right call, and what many consider to be one of the most important decisions in the entire war.
Weather stations in Ireland gave the Allies the upper hand, giving them the necessary data and current conditions to make that crucial decision to delay the invasion.
Cloudy skies would have made it difficult for aircraft to carry out air support. Rough seas and wind could knock over and capsize boats, and if it rained, the Army could have a rough time as well.
President Eisenhower agreed to suspend operations. A new date was set for June 6th, 1944.
The weather wasn't very cooperative to the Allies on the morning of June 6th. As the History Channel notes, "Thick clouds resulted in Allied bombs and paratroopers landing miles off target. Rough seas caused landing craft to capsize and mortar shells to land off the mark. By noon, however, the weather had cleared and Stagg’s forecast had been validated. The Germans had been caught by surprise, and the tide of World War II began to turn."
Operation Overlord became the largest seaborne invasion in history.