The Normandy Invasion was the invasion and establishment of forces from the Allies (United States of America, United Kingdom, Canada and other nations) in Normandy, France during Earth's World War II. Using the codename D-Day, the invasion began on June 6, 1944. The overall invasion was known as Operation Overlord.
The responsibility for planning and supervising the invasion was held by General Dwight D. Eisenhower, who served as Supreme Commander of the Allied forces in Europe. (TOS - Star Trek II Short Stories short story: "The Blaze of Glory")
A Changeling travelled back in time to just before the invasion and attempted to change history by warning Adolf Hitler of the invasion. He was stopped by Jake Sisko and Nog. They were assisted by Brigitte Picard, an ancestor of Jean-Luc Picard. (DS9 novel: Trapped in Time)