Germany had invaded Poland in 1939, and had occupied France after May, 1940. The Western Allies tried to resist the German advance into France in 1940, but were forced to evacute as many soldiers as possible from the port of Dunkirk in late may and early June. had planned to retake Europe. Their first attempt, Operation Jubilee, commonly referred to as the Dieppe Raid in August 1942, was a dismal failure. However, the Allies learned several critical lessons about what not to do and what to do instead when attacking a coastal defence system.
By 1944 the Allies were planning to establish a beachhead in France from which they could move through the Rhineland and into the heart of Germany, and thus bring the war to a speedier conclusion. Winston Churchill thought that the Allies were not yet ready to attack Germany via France, and wanted to conduct a battle of attrition by attacking Germany on several fronts including via “the soft underbelly” of Europe, through Italy. The Allies had landed at Sicily and had worked their way up through Italy as far as Rome, when the time was deemed to be right for an attack through France.
Operation Overlord was the codename for the Battle of Normandy, and it started with Operation Neptune, which was the landings on June 6, 1944, commonly known as “D-Day”.