Operation Overlord was the code name for the Battle of Normandy, the operation that launched the invasion of German-occupied western Europe during World War II by Allied forces. The operation commenced on 6 June 1944 with the Normandy landings (Operation Neptune, commonly known as D-Day). A 12,000-plane airborne assault preceded an amphibious assault involving almost 7,000 vessels. Nearly 160,000 troops crossed the English Channel on 6 June; more than three million troops were in France by the end of August.
Allied land forces that saw combat in Normandy on D-Day itself came from Canada, the United Kingdom and the United States. Free French Forces and Poland also participated in the battle after the assault phase, and there were also minor contingents from Belgium, Greece, the Netherlands, and Norway. Other Allied nations participated in the naval and air forces.
Once the beachheads were secured, a three-week military buildup occurred on the beaches before Operation Cobra, the operation to break out from the Normandy beachhead, began. The battle for Normandy continued for more than two months, with campaigns to expand the foothold on France, and concluded with the closing of the Falaise pocket on 24 August, the Liberation of Paris on 25 August, and the German retreat across the Seine which was completed on 30 August 1944. --Wiki