The unrestricted U-Boat war threatened the very survival of Britain, whose reliance on imported food and war materials was her Achilles' Heel. A significant element of the German submarine fleet operated from the occupied Belgian ports of Zeebrugge and Ostend. After careful planning the Royal Navy launched audacious attacks on these two ports on St Georges' Day 1918. Five obsolete cruisers and two Mersey ferries supported by a flotilla of smaller vessels penetrated the near impregnable defences, while Royal Marines and naval storming parties battle ashore in a diversionary attack. At the time of the action the concrete filled block ships were scuttled in the ports' approaches. Despite being a costly and bloody affair for the participants, the survivors returned to acclaim. The raids gave a fillip to the national morale, at a time of depressing news from the Front. To underline the success of the affair no less than 11 Victoria Crosses were awarded.