On D-Day, 6 June 1944, the Allies of World War II launched Operation Overlord, the invasion of Normandy. They achieved tactical and operational surprise, and established a lodgement. In the weeks that followed, the Germans made skilful use of the difficult and defensible terrain of the "bocage" country, and the Allied advance was slower and more costly than anticipated. A storm on 19 to 21 June wrecked the Mulberry harbour at Omaha Beach and gave a foretaste of the devastation that the autumn gales could bring. There was a genuine prospect of a World War I-style stalemate developing if the German defenders could hold out until then. But the German decision to hold on in Normandy also allowed the Allies to employ their superiority in firepower, subjecting the Germans to casualties and destruction of equipment on a scale that they could not replace, thereby making their ultimate defeat in Normandy inevitable. On 25 July, the Allies launched Operation Cobra, and broke out from the lodgement in Normandy. By mid-August 1944, the Allies had not yet crossed the Seine, but the Germans were in full retreat. Indeed, they were retreating on all fronts: in Southern France, where the Allies had landed on the French Riviera; in Italy, where the Germans were falling back from Rome to the Gothic Line; and in Belarus, where a German army group had been broken by Soviet offensive Operation Bagration.