The submarine was undoubtedly the most potent purely naval weapon of the twentieth century. In two world wars, enemy underwater campaigns were very nearly successful in thwarting Allied hopes of victory—indeed, annihilation of Japanese shipping by US Navy submarines is an indicator of what might have been. That the submarine was usually defeated is a hugely important story in naval history, yet this is the first book to treat the subject as a whole in a readable and accessible manner. It concerns individual heroism and devotion to duty, but also ingenuity, technical advances and originality of tactical thought. What developed was an endless battle between forces above and below the surface, where a successful innovation by one side eventually produces a counter-measure by the other in a lethal struggle for supremacy. Development was not a straight line: wrong ideas and assumptions led to defeat and disaster.