Perhaps the most critical battle of WWII was the one the Allied forces waged to keep the trade routes open. Food and raw materials being essential to the war effort, without which the end result would surely have been different. The German U-boat was a hidden predator, stealing up on its prey out of sight bencath the waves, more effective when roaming in groups until it chose to attack. Admiral Donitz chose to use the grey hulls of his submarines in these 'wolf packs'. The Allied defense against this simple yet stunning strategy was fraught with difficulty and huge Allied losses were inflicted. The depths of German submarinal action are delved into, to explain this remarkable naval tactic, cunningly used with great brutality by men who themselves withstood awesome deprivation, securing the adulation of their nation and very nearly winning the Battle of the Atlantic.