The Three Power Alliance And The United States-Japanese War, written by Matsuo while he was serving as a liaison between the Japanese Foreign Office and the Admiralty, openly discussed the impending hostilities.
In the years prior to the attack on Pearl Harbor on December 7, 1941, Japan constructed an ocean fortress behind a wall of secrecy in violation of its diplomatic agreement with the League.
The mandated islands, including the Northern Marianas, were forbidden territory to U.S. ships and American naval authorities were becoming increasingly apprehensive over Japan's rearmament and the growing belligerency of its military, first overtly observed in the Panay incident.
Other reports indicated that the previously mentioned book was an important secret Japanese high-level document, and was alleged to have been stolen from Kinoaki Matsuo, a high-ranking officer of the Black Dragon Society by Kilsoo Haan, an anti-Japanese Korean patriot.
[1] This document was purported to detail Japanese war plans for the simultaneous invasion of the Panama Canal Zone, Alaska, California and Washington.