Excerpts from Dr. Carroll Quigley's Book Tragedy and Hope on the topic of Pearl Harbor:
Espionage is another matter, but this is more from the nature of espionage than the nature of Communism, except for the very significant fact that the ideological appeal of Communism to the half educated makes it possible for the Soviet Union to obtain secrets without financial payments.
In general, the nature of espionage is totally ignored by most people, and this ignorance was only increased by the activities of the anti-Communist spy agitations of the 1949-1954 period.
All past history shows that espionage has been generally successful and intelligence has been generally a failure.
By this I mean that no country had much success in keeping secrets, in the twentieth as in all earlier centuries, but neither has any other country had much success in evaluating or in interpreting the secrets it obtained.
The so-called "surprises" of history have emerged not because other countries did not have the information but because they refused to believe it.
The date of Hitler's attack on the West in May 1940 had been given to the Netherlands by the German Counterintelligence Office as soon as it was decided; the Western countries refused to believe it.
The same was true of every one of Hitler's surprises. Stalin was given the date of the German attack on the Soviet Union by a number of informants, including the United States Department of State, but he refused to believe. Both the Germans and the Russians had the date of D-Day, but ignored it. The United States had available all the Japanese coded messages, knew that war was about to begin, and that a Japanese fleet with at least four large carriers was loose (and lost) in the Pacific, yet Pearl Harbor was a total surprise. This last point was so hard to believe, once the evidence was available, that the same groups who were howling about Soviet espionage in 1948-1955 were also claiming that President Roosevelt expected and wanted Pearl Harbor.
Both these beliefs, if they were believed, were based on gigantic ignorance and misconceptions about the nature of intelligence.
The whole purpose of secrecy in government should not be to keep information from other states (this is almost impossible) but to make it as difficult as possible for other states to get certain information, so that, when they do get such restricted information, it will be so intermingled with other information and misinformation that it cannot be evaluated promptly enough to do them much good.
Any espionage system gets more information than it can handle rapidly. Any country should assume that the enemy has all its own secret information. The lessons of past history fully support this assumption. Following every war the discovery is made that the enemy, during the war, had every other state's most cherished secrets. In fact, the most successful kind of counterespionage work is achieved, not by preventing access to secrets, but by permitting access to information which is not true.
This was done most successfully in 1943 in preparing the American invasion of Sicily, which was a surprise to the Germans because they had been provided, through their espionage in Spain, with false information about an invasion of the Balkans. The Germans had a somewhat similar success through their Operation North Pole by which the Germans successfully took over and operated the French Underground and the associated British espionage net in a large part of France for about a year.
Finally, it is not generally recognized by outsiders that almost all the information gathered by any espionage net is non-secret material fully available to anyone as public information. Even in work against a super-secret area like the Soviet Union or in nuclear "secrets" this is true. Allen Dulles said that more than go percent of the information which the CIA gathers on the Soviet Union is non-secret. Soviet espionage reports on the United States must contain at least 97 percent non-secret material.http://www.carrollquigley.net/books.htm