The D-Day Map Room at Southwick House. On D-Day, 6 June 1944, Allied forces launched a combined naval, air and land assault on Nazi-occupied France. Codenamed Operation 'Overlord', the Allied landings on the Normandy beaches marked the start of a long and costly campaign to liberate north-west Europe from German occupation. The operation was planned and coordinated in this room and the original map can still be seen on the wall today, preserved just as it was in 1944.