In late 1940 members of the U.S. Army Signal Intelligence Service broke Japan's highest level diplomatic code and then, using only analytic means, constructed an analog of the code machine used by the Japanese. This allowed the U.S. to read Japan's diplomatic traffic from then until after the end of the war. Intelligence thus gained was cover named MAGIC because it seemed that only magicians could have produced it. Because of its sensitivity it was only available to a select few of the highest-level leaders in our government.
While there was considerable disagreement in the 1942 government regarding the necessity for Japanese evacuation from the West Coast, not one person with access to MAGIC argued against the action.
The messages presented here are a selection from about a hundred that are reproduced in the book that reveal Japans intentions and requirements for intelligence on the U.S. in 1941.