This WWII history chronicles the remarkable engineering achievement that kept vital supplies flowing to Allied forces after D-Day. In the planning stages of the Normandy invasion, Allied strategists correctly anticipated that the Germans would deny, either by destruction or dogged defense, the vital Channel ports in the aftermath of D-Day. If the invading armies could not be kept resupplied, Operation Overlord would fail. The only solution was to design, build, transport and install two massive artificial harbors.