D-Day (military term) in the context of "Invasion of Normandy"

The Normandy landings were the landing operations and associated airborne operations on 6 June 1944 of the Allied invasion of Normandy in Operation Overlord during the Second World War. Codenamed Operation Neptune and often referred to as D-Day (after the military term), it is the largest seaborne invasion in history. The operation began the liberation of France, and the rest of Western Europe, and laid the foundations for the Allied victory on the Western Front.

Planning for the operation began in 1943. In the months leading up to the invasion, the Allies conducted a substantial military deception, codenamed Operation Bodyguard, to mislead the Germans as to the date and location of the main Allied landings. The weather on the day selected for D-Day was not ideal, and the operation had to be delayed 24 hours; a further postponement would have meant a delay of at least two weeks, as the planners had requirements for the phase of the moon, the tides, and time of day, that meant only a few days each month were deemed suitable. German leader Adolf Hitler placed Field Marshal Erwin Rommel in command of German forces and developing fortifications along the Atlantic Wall in anticipation of an invasion. US president Franklin D. Roosevelt placed Major General Dwight D. Eisenhower in command of Allied forces.

The Battle of Saint-Mihiel was a major World War I battle fought from 12 to 16 September 1918, involving the American Expeditionary Forces (AEF) and 110,000 French troops under the command of General John J. Pershing of the United States against German positions. The U.S. Army Air Service played a significant role in this action.

This battle marked the first use of the terms "D-Day" and "H-Hour" by the Americans.