The operation would happen on June 6, 1944, but what would become the largest amphibious assault in history had been in the works for years. Preparation began in December 1943 when President Franklin D. Roosevelt named General Dwight D. Eisenhower the Supreme Commander of the Allied Expeditionary Force, notes the National World War II Museum.
You might think the “D” in D-Day stands for “Designated” or “Decision,” but the D actually stands for “Day,” because it is a coded designation used for the day of any important invasion or military operation. So actions taken four days ahead of the actual operation, for instance, would be D-4, according to the U.S. Army.
Operation Overlord was the code name for the D-Day invasion. Overlord included all of the planning that went into the assault, which would involve the transporting of more than 150,000 infantry troops across the English Channel into German-occupied France.
By the end of May 1944, more than 1.5 million U.S. Army personnel had arrived in the U.K. to participate or support the operation, according to the National World War II Museum. Overall, more than two million soldiers from the U.S. soldiers and 250,000 from Canada had arrived by June in preparation for, and to support, the invasion, according to History.com. Also delivered: 7 million tons of supplies including 450,000 tons of ammunition.
Operation Overlord also contained a fake operation called Operation Fortitude to convince Hitler the Allies would attempt to land in Norway and Pas-de-Calais in France where the English Channel is narrowest. The plot, concocted over months, included a fake army, led by General George Patton, and readying in Dover for the channel crossing, notes History.com.
