Ultra was the high-grade intelligence made available to the Allies throughout WW II, thanks to the UK's ability to read many of the Wehrmacht's Enigma ciphers. Messages sent via Kriegsmarine systems, however, were appreciably tougher to decode than those from Enigma machines employed by other branches of the Nazi military. As one result, Hitler's U-boats took a heavy toll on merchant shipping, threatening Great Britain's high-seas lifeline during the early years of the war. Newsday editor Kahn (Kahn on Codes, 1983) offers a wide-ranging appreciation of how the Royal Navy furnished the Oxbridge dons and other boffins posted to England's Bletchley Park the material they needed to decipher submarine signals. In brief, the high command authorized a series of attacks on German weather vessels gathering climatic data offshore Iceland. These forays, plus the fortuitous capture of several U-boats, paid off in up-to-the-minute rundowns on code-wheel settings, which allowed cryptanalysts to read tactical communiquÇs almost as quickly as sub captains. Consequently, the Admiralty was able to route convoys away from wolfpacks, saving untold numbers of vessels and keeping desperately needed supplies moving from North America to the island nation. Among other fresh perspectives, Kahn provides detailed, action-packed accounts (drawn from interviews with surviving eyewitnesses on both sides) of the bold seizures that yielded vital documents. Covered as well are the contributions of Polish mathematicians to unriddling Enigma transmissions, code-breaking successes by the Axis, German countermeasures, and the intricacies of convoy management. The author is as pains to stress that Ultra intelligence, while important for helping to save blood and treasure, was not decisive in the battle of the Atlantic. As the conflict intensified, he concludes, Anglo-American forces gained an unbeatable edge in technology and firepower; their shipyards, moreover, produced replacement bottoms at a pace faster than U- boats could sink operational fleets. A first-rate briefing on the use of brawn as well as brains to alleviate the U-boat threat.