Gambler, thief and rotter, but no traitor

David Kahn, an independent scholar, discusses in this book the life and work of a "cult figure". H.O. Yardley was in the decade to 1929 America's leading codebreaker. Kahn argues that Yardley established his trade as an accepted peacetime activity of the US Government.

He was able to enhance the standing of his profession because his unit, the "American Black Chamber" (ABC), cracked the codes Tokyo used to communicate with its negotiators at the 1921 Washington naval disarmament conference, at which Japan was driven down from a 10:7 Pacific tonnage ratio to 10:6. Kahn acknowledges that the ABC worked slowly and without unique insight: for example, The New York Times had already revealed that the Japanese were going to soften their negotiating stance.

But he insists that it was because the ABC cracked a message indicating that Japan would compromise, that Secretary of State Charles Evans Hughes mustered the fortitude to force Tokyo's concession.

In 1929, Secretary of State Henry L. Stimson dissolved the ABC on the legendary grounds that "gentlemen do not read each other's mail". This, and the Wall Street crash of that year, meant Yardley was out of a job. He now allegedly sold American codebreaking secrets to Japan, and he also published The American Black Chamber , a book revealing national security secrets.

The Boston Post reported the widespread view that Yardley's actions amounted to treachery, but Kahn believes Yardley was "a rotter, not a traitor". He concedes that Yardley drank heavily, ran after "skirt" and was greedy, and is ready to admit that his main skills lay in promotion, not codebreaking. But he denies that Yardley's irresponsibility caused the Japanese to change their codes, leading to the success of the surprise attack on Pearl Harbor. Japan, Kahn argues, would have changed its codes anyway, and was already moving towards automated encryption.

Up to a point, Kahn takes a clear-eyed view. Sometimes, one can almost see a self-deprecating twinkle in his eye, for example when he refers to "practical cryptanalysis", the art of stealing other people's code books.

But at critical junctures he gives an impression of sleepwalking on hallowed ground. Codebreaking, he claims, is the most valuable form of intelligence. His faith in the art is reminiscent of British intelligence historian Fred Hinsley's claim that Bletchley Park shortened the Second World War by several years. Such claims are pure conjecture and disregard other factors such as Japan's strategic maturity in 1921, the military impact of the US's entry into the 1939-45 conflict and the work of human agents.

Kahn's observation that Yardley "won at poker" betrays an adulatory element in his approach to a man who, he admits, had unsavoury characteristics. To be sure, Yardley wrote The Education of a Poker Player , a minor classic in the art of bluff. But, like all addictive gamblers, in the long run he lost heavily. Indeed, it was to service his debt that he engaged in one dubious scheme after another. Only because of the perception that the US needs an intelligence icon does the deeply flawed Yardley remain on his pedestal.

Though Kahn writes with lucidity about the process of codebreaking, he has a muddled appreciation of Yardley the man and of his historical significance.

Rhodri Jeffreys-Jones is professor of American history, Edinburgh University.