Normandy Invasion preparations, 1944. Concrete Phoenix caisson is towed into an English harbor, following its construction by the Portsmouth Dockyard, England, 16 March 1944. These caissons were built for use as breakwaters for the Mulberry artificial harbors that were part of the Normandy invasion plan. The D-Day landings of 6 June 1944 were the largest seaborne invasion in history. They successfully liberated the Normandy coast of Nazi-occupied France, contributing to the Allied victory in World War II.