"Operation Overlord" was the codename for the Allied invasion of Hitler's Fortress Europe on the beaches and fields of Normandy, France. Otherwise known as "D-Day", the invasion was, and still is, the largest sea borne invasion in military history. Over 850,000 troops were landed in the months following the day of the invasion, June 6th, 1944.
The objective of this invasion was to gain a foothold in Europe so the Allied Expeditionary Force could land and fight the Axis forces in northern Occupied France. The Allies already had forces in Southern Europe and Italy, but the extremely heavy fighting at Monte Casino and the inability to pass through the Italian Alps halted their advance. The plan was set in motion in late 1943 by COSSAC (Chief of Staff to the Supreme Allied Commander).
The landings took place along the beaches of the Normandy coastline. These beaches were at the heart of Fortress Europe. The name was given to the extremely fortified emplacements and defenses along the coastline. The "Atlantic Wall", as it became to be known, was thought to be impenetrable by people on both sides of the fighting. The wall consisted of a massive, World War I like trench system, concertina (barbed wire entanglements), beach obstacles, hundreds of machine-gun nests and pillboxes, minefields, bunkers containing mortars and concrete bunkers containing heavy artillery including 66mm, 75mm, 88mm, 115mm, and 155mm gun emplacements. The wall was manned by troops of the Ost (East) Battalion. Fifty percent of the battalion was made up of captured Polish and Russian soldiers that fought only so they would not be shot by the sergeants holding guns to their heads. Behind the wall, hundreds of fields of farmland were flooded to prevent airborne landings. The armored units, anti-aircraft emplacements, and reserve troops were uncoordinated, and disorganized. These and other mistakes made by the German military gave the Allies several advantages.