From Chapter 7 ("Surprise") of Roberta Wohlstetter's Pearl Harbor, good comments on hindsight and the false illusions it encourages:

... it is much easier after the event to sort the relevant from the irrelevant signals. After the event, of course, a signal is always crystal clear; we can now see what disaster it was signaling, since the disaster has occurred. But before the event it is obscure and pregnant with conflicting meanings. It comes to the observer embedded in an atmosphere of "noise," i.e., in the company of all sorts of information that is useless and irrelevant for predicting the particular disaster. For example, in Washington, Pearl Harbor signals were competing with a vast number of signals from the European theater. These European signals announced danger more frequently and more specifically than any coming from the Far East. The Far Eastern signals were also arriving at a center of decision where they had to compete with the prevailing belief that an unprotected offensive force acts as a deterrent rather than a target. In Honolulu they were competing not with signals from the European theater, but rather with a large number of signals announcing Japanese intentions and preparations to attack Soviet Russia rather than to move southward; here they were also competing with expectations of local sabotage prepared by previous alert situations.

In short, we failed to anticipate Pearl Harbor not for want of the relevant materials, but because of a plethora of irrelevant ones. Much of the appearance of wanton neglect that emerged in various investigations of the disaster resulted from the unconscious suppression of vast congeries of signs pointing in every direction except Pearl Harbor. It was difficult later to recall these signs since they had led nowhere. Signals that are characterized today as absolutely unequivocal warnings of surprise air attack on Pearl Harbor become, on analysis in the context of December, 1941, not merely ambiguous but occasionally inconsistent with such an attack. ...

^z - 2017-12-05