The Mulberry Harbours were used during the Second World War as part of Operation Overlord, the invasion of northern Europe by the Allies in June 1944. This commenced with the D-Day landings on the Normandy beaches on 6th June. The harbours played an important role in the history of ocean engineering leading to the development of novel technology and new theory. A severe storm occurred soon after the harbours were deployed leading to the destruction of the American harbour and severe damage to the British one. In this paper, we analyse this storm using hindcast data from ECMWF and SWAN modelling. We find that the waves were significantly more severe at the American harbour than at the British one, which may partially explain why the latter experienced less damage. We also find that the usually quoted figure for the storm severity of 1 in 40 years is a reasonable estimate for a summer storm at these locations.