Rear Admiral Edwin T. Layton has argued that the lack of a Purple machine in Hawaii prevented Admiral Husband Kimmel and General Walter Short, the commanders there, from using Magic-provided information on international affairs to illuminate their situation. This would have enabled them to predict the attack, Layton has claimed. But this is speculation, supported only by hindsight. Moreover, the presence of a Purple machine in the Philippines did not prevent the American forces from being surprised.

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Author John Toland found several former radio operators who had been listening in San Francisco or at sea. They said that in the week before December 7, they had heard a cacophony of radio signals from northwest of Hawaii–presumably the strike force heading for Pearl Harbor. They said they reported this, to no effect. But this story founders because, according to the Japanese, the strike force maintained absolute radio silence throughout its voyage. And American naval intercept units, straining to pick up whatever they could on the Japanese naval circuits, heard nothing. U.S. radio-intelligence operators knew that several carriers had dropped out of the traffic picture. They thought that the ships were in home waters, covering a movement to the south–the Philippines or the oil- and rubber-rich Dutch East Indies. Carrier communications had likewise vanished in February and July 1941, and naval radio-intelligence operators hypothesized then that the carriers had been held near Japan–a hypothesis later determined to be factual. But what happened then was not what was happening in December.

Several writers have suggested that the solution of the many messages dealing with ship movements in and out of Pearl Harbor should have alerted the authorities to the impending attack. But similar messages were transmitted about the Philippines, the Panama Canal, San Francisco, San Diego and Seattle. In fact, from August 1 to December 6, 59 such intercepts dealt with the Philippines and only 20 with Hawaii. The writers have pointed to one intercept, instructing the consulate in Hawaii to divide the Pearl Harbor anchorage into smaller areas for more precise reporting of ship locations, as a clear indication of a forthcoming attack. That is hindsight. At the time, the authorities viewed it merely as evidence of the thoroughness of Japanese intelligence or of the need to abbreviate communications.

James Rusbridger, in his book Betrayal at Pearl Harbor, claims that the British code-breaking unit at Singapore solved enough of JN25 to reveal the plan to attack Pearl Harbor and that this information was passed on to Prime Minister Winston Churchill, who withheld it from Roosevelt to ensure American entry into the war, thereby enabling the attack to succeed. Citing the Official Secrets Act, British authorities have denied Rusbridger any access to the records of the unit, and the U.S. Navy reports that it cannot find any of these JN25b solutions, partial or complete, from before December 7. His thesis rests on the memory of an Australian code-breaker working in Singapore. That is a slender reed upon which to base so heavy a charge. Moreover, Rusbridger does not distinguish between the a and b editions of JN25–and does not make clear why Churchill would try to get the Americans to fight Japan instead of Germany.

A retired communications intelligence analyst, Fred Parker, has dredged through the files of Japanese messages intercepted before Pearl Harbor but not solved until afterward. Though he has found no smoking gun, no message referring specifically to an attack on Pearl Harbor, he believes that the messages he has found point clearly to an impending attack there. He cites, for example, the presence of an oiler on what became the strike force's homeward route and the broadcast of the message 'Climb Mount Niitaka 1208.' The oiler obviously was put there, he says, to refuel the returning ships. The 1208 in the message meant December 8, the attack date on the Tokyo side of the international date line. Mount Niitaka (Hsin-kao in Taiwan) was the highest peak in what was then the Japanese empire. Parker contends that a solution of these messages would have suggested an attack on Pearl Harbor. Like the other theories, however, this is hindsight.

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