The Ambassador in Japan (Grew) to the Secretary of State
Tokyo, November 17, 1941—1 p. m.
Received November 17—2:09 p. m.)
1814. Referring to Embassy's previous telegram No. 1736 of November 3, 3 p. m., final sentence, and emphasizing the need to guard against sudden Japanese naval or military actions in such areas as are not now involved in the Chinese theater of operations. I take into account the probability of the Japanese exploiting every possible tactical advantage, such as surprise and initiative. Accordingly you are advised of not placing the major responsibility in giving prior warning upon the Embassy staff, the naval and military attaches included since in Japan there is extremely effective control over both primary and secondary military information. We would not expect to obtain any information in advance either from personal Japanese contacts or through the press the observation of  military movements is not possible by the few Americans remaining in the country, concentrated mostly in three cities (Tokyo, Yokohama, Kobe); and with American and other foreign shipping absent from adjacent waters the Japanese are assured of the ability to send without foreign observation their troop transports in various directions. Japanese troop concentrations were reported recently by American consuls in Manchuria and Formosa, while troop dispositions since last July's general mobilization have, according to all other indications available, been made with a view to enabling the carrying out of new operations on the shortest possible notice either in the Pacific southwest or in Siberia or in both.
We are fully aware that our present most important duty perhaps is to detect any premonitory signs of naval or military operations likely in areas mentioned above and every precaution is being taken to guard against surprise. The Embassy's field of naval or military observation is restricted almost literally to what could be seen with the naked eye, and this is negligible. Therefore, you are advised, from an abundance of caution, to discount as much as possible the likelihood of our ability to give substantial warning.
(PROCEEDINGS OF CLARKE INVESTIGATION pg. 150)