From Chapter 7 ("Surprise") of Roberta Wohlstetter's Pearl Harbor:

... The fact of surprise at Pearl Harbor has never been persuasively explained by accusing the participants, individually or in groups, of conspiracy or negligence or stupidity. What these examples illustrate is rather the very human tendency to pay attention to the signals that support current expectations about enemy behavior. If no one is listening for signals of an attack against a highly improbable target, then it is very difficult for the signals to be heard.

For every signal that came into the information net in 1941 there were usually several plausible alternative explanations, and it is nor surprising that our observers and analysts were inclined to select the explanations that fitted the popular hypotheses. They sometimes set down new contradictory evidence side by side with existing hypotheses, and they also sometimes held two contradictory beliefs at the same time. We have seen this happen in G-2 estimates for the fall of 1941. Apparently human beings have a stubborn attachment to old beliefs and an equally stubborn resistance to new material that will upset them

Besides the tendency to select whatever was in accord with one's expectations, there were many other blocks to perception that prevented our analysts from making the correct interpretation. We have just mentioned the masses of conflicting evidence that supported alternative and equally reasonable hypotheses. This is the phenomenon of noise in which a signal is embedded. Even at its normal level, noise presents problems in distraction; but in addition to the natural clatter of useless information and competing signals, in 1941 a number of factors combined to raise the usual noise level. First of all, it had been raised, especially in Honolulu, by the background of previous alert situations and false alarms. Earlier alerts, as we have seen, had centered attention on local sabotage and on signals supporting the hypothesis of a probable Japanese attack on Russia. Second, in both Honolulu and Washington, individual reactions to danger had been numbed, or at least dulled, by the continuous international tension.

A third factor that served to increase the natural noise level was the positive effort made by the enemy to keep the relevant signals quiet. The Japanese security system was an important and successful block to perception. It was able to keep the strictest cloak of secrecy around the Pearl Harbor attack and to limit knowledge only to those closely associated with the details of military and naval planning. In the Japanese Cabinet only the Navy Minister and the Army Minister (who was also Prime Minister) knew of the plan before the task force left its final port of departure.

In addition to keeping certain signals quiet, the enemy tried to create noise, and sent false signals into our information system by carrying on elaborate "spoofs." False radio traffic made us believe that certain ships were maneuvering near the mainland of Japan. The Japanese also sent to individual commanders false war plans for Chinese targets, which were changed only at the last moment to bring them into line with the Southeastern movement.

A fifth barrier to accurate perception was the fact that the relevant signals were subject to change, often very sudden change. ...

Sixth, our own security system sometimes prevented the communication of signals. It confronted our officers with the problem of trying to keep information from the enemy without keeping it from each other, and, as in the case of MAGIC, they were not always successful. ...

To these barriers of noise and security we must add the fact that the necessarily precarious character of intelligence information and predictions was reflected in the wording of instructions to take action. The warning messages were somewhat vague and ambiguous. ... The fact that intelligence predictions must be based on moves that are almost always reversible makes understandable the reluctance of the intelligence analyst to make bold assertions. Even if he is willing to risk his reputation on a firm prediction of attack at a definite time and place, no commander will in turn lightly risk the penalties and costs of a full alert. ...

Last but not least we must also mention the blocks to perception and communication inherent in any large bureaucratic organization, and those that stemmed from intraservice and interservice rivalries. ... A general prejudice against intellectuals and specialists, not confined to the military but unfortunately widely held in America, also made it difficult for intelligence experts to be heard. ...

^z - 2017-12-12