An excellent short history--tight and sharp--of how the German Enigma machine was obtained, how its codes were deciphered, what effect its revelations had on the course of World War II. Calvocoressi, an activist publisher (Freedom to Publish, p. 1373) and practiced historian (Total War, 1972), worked during the war at Bletchley Park, the British center for decoding activities; he is at ease with the technicalities and up on internal operations. Unlike the divided and disorganized German cryptographic efforts, he points out, the Allied effort was ""inter-service and non-service""; each scrap of information, consequently, could be exploited to the fullest. Provided with an Enigma machine by the Poles (who had monitored German signals from 1933 to 1938), the Allies scored their first gain when the German code for the invasion of Norway was cracked. This was followed by the reading of the Luftwaffe codes (""more frequently and continuously broken than others""); of the German naval codes (instrumental in the sinking of the Bismarck); and, most decisively, of the various U-boat codes--without which ""the Battle of the Atlantic would have been won by Doenitz."" Wehrmacht codes, on the other hand, proved exceedingly difficult to crack--a disadvantage offset by the continual reading of radio traffic from Luftwaffe army liaison officers, providing information on the ground forces they served with. Among other strategic boom, Calvocoressi cites Churchill's ability to send precious resources to the Middle East when Ultra revealed that the planned invasion of Europe had been postponed. (Stalin turned a deaf ear, however, to his Ultra-inspired warning of German preparations for the invasion of the USSR.) Most typically, Calvocoressi reports, Ultra provided ""a constant and thorough delineation of the German order of battle."" His book is more succinct and (because of his participation) more immediate than Ronald Lewin's 1979 Ultra Goes to War, though not significantly more informative.