Ultra, the name given to information gained from the deciphering of messages passed by the enemy in code during the Second World War, was certainly an invaluable asset to the Allies; but its application gave rise to other problems. If the information gained was used too freely, the enemy would soon suspect that their radio traffic was being intercepted and read, and would take the necessary counter-measures.

Fortunately for the Americans in the Pacific, the Japanese sincerely believed that it was not possible for Westerners to learn their language. Lulled by this misapprehension into a false sense of security, they could only ascribe to luck or coincidence the remarkable frequency with which the Americans intercepted their plans.

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The war in the Pacific has had many chroniclers but the secret of Ultra remained guarded for many years and only recently has it become possible to assess in detail the effect it has on the campaign. John Winton's expert analysis of the records now available are here combined with his encyclopedic knowledge of the naval history of the Second World War to tell, for the first time, what exactly the Allies did learn from Ultra in the Pacific War and to what use that knowledge was put.

The result is a fascinating story told with the zest and pace one might expect from an author who is both a highly respected historian and a first-class novelist. His long-overdue tribute to Joe Rochefort, whose contribution to the successful outcome of the War in the Pacific is hard to overemphasize, is particularly welcome.