D-Day, also known as Operation Overlord, was a military operation during World War II in which Allied forces invaded Nazi-occupied Normandy, France on June 6, 1944. It was the largest seaborne invasion in history and marked a turning point in the war. The operation involved over 156,000 troops, 5,000 ships, and 11,000 aircraft, and resulted in the establishment of a foothold in France that eventually led to the liberation of Europe from Nazi control.