If Hitler had not declared war on the United States following Pearl Harbor, it would have been a master stroke. There was no certainty that the American people would have called for a declaration of war against Germany, and Americans and British would have been trapped in a war in the Far East that would have divided British strength and diverted American arms and supplies from the European front.
Paradoxically, there was no shortage of intelligence about an impending war with Japan; the problem lay in the fact that there was so much information that policy-makers found it impossible to unravel Japanese intentions. Most of this intelligence was conflicting. The conventional wisdom held that the Japanese would try to seize the resources of Southeast Asia and perhaps attack the Philippines. Other analysts predicted an attack on the South Soviet Union. A Pearl Harbor attack was ruled out because it was believed the Japanese lacked the capacity to mount such an operation.
Racism had much to do with this as lack of foresight. Americans regarded the Japanese as bucktoothed, bespectacled little men, always photographing things with their ever-present cameras so they could copy them. Japanese planes and ships were said to be inferior copies of American models, myopic Japanese pilots would be unable to hit their targets and Japan's teahouse economy would quickly collapse under wartime strain. The New York tabloid PM ran an article on "How We Can Lick Japan in Sixty Days."
Information poured in upon Washington from a variety of sources. American code experts had succeeded in breaking the supposedly impregnable "Purple" diplomatic traffic between Tokyo and its emissaries in Washington. Adding to the cornucopia were messages in the lower-priority J-19 code, the tracking of Japanese naval vessels by their call signs, reports from American diplomats in Tokyo and the observed movements of Japanese troops and vessels. Random clues to the Pearl Harbor raid were embedded in this mass of information, but the volume was so overwhelming that cryptanalysts were unable to determine immediately the significant from the irrelevant.
Only later, in the brilliant light of hindsight were the relevant hints seen as heralds of a forthcoming Japanese attack. For example, not long after Admiral Yamamoto began the secret planning for the Pearl Harbor raid, Joseph C. Grew, the U.S. envoy to Japan, notified Washington that the Peruvian minister had learned "from many sources, including a Japanese source, that in the event of war breaking out between the United States and Japan, the Japanese intended to make a surprise attack against Pearl Harbor." The information was relayed to Kimmel -- with an assessment that read: "Naval Intelligence places no credence in these rumors."
"After an event a signal is always crystal clear," observes Roberta Wohlstetter in "Pearl Harbor: Warning and Decision," her account of the intelligence background of Pearl Harbor. "We can see what disaster it was signaling, since the disaster had occurred. But before the event it is obscure and pregnant with conflicting meanings . . . In short, we failed to anticipate Pearl Harbor not for want of relevant materials, but because of a plethora of irrelevant ones."
One of the major hazards of intelligence work is the tendency to rely too heavily upon a single source. Roosevelt and his associates believed that MAGIC -- the code-breaking operation reading the Japanese diplomatic code -- provided them with an infallible key to the maze of Japanese intentions. Other sources were downgraded or ignored. But Purple revealed only the information being passed on by the Foreign Office to its key representatives abroad. The most secret Japanese naval code, JN-25, in which vital naval signals were transmitted, had not been broken by the Americans. (Recently, there have been reports that the British broke JN-25 before Pearl Harbor, but Prime Minister Winston S. Churchill did not inform Roosevelt of the fact in order to tumble America into the war.)
Thus, MAGIC was a double-edged sword. While it gave American policy-makers inside knowledge of Japanese intentions, it created overconfidence. MAGIC was limited to only the president, the secretaries of State, War and Navy and few top military officers. This fetish for security was self-defeating. Neither Kimmel nor Short was privy to MAGIC, which would have allowed them to monitor the progress of the Japanese-American negotiations under way in Washington. A Purple machine was to have been sent to Pearl Harbor but it was given the British, instead.