But in these two years, the RAF learned what it needed to win the Battle of the Atlantic. Gradually developing new tactics and technology, such as airborne radar, signals intelligence, and effective weaponry, the Allies ended 1941 in a position to defeat Donitz's growing fleet of U-boats. This book, the first of two volumes, explains the fascinating history of how the RAF kept the convoys alive against the odds, and developed the force that would prevail in the climactic battles of 1942 and 1943.