One of the greatest contributions of the Military Intelligence Service (MIS) during World War II was the translation of the captured "Z Plan." The "Z Plan" contained Japan's defensive strategy against the Allies for its southern Pacific possessions and its strategy for an all-out attack against the American fleet in the Pacific, specifically in the Mariana Islands.
The Mariana Islands, an archipelago of 15 volcanic islands located southwest of the Hawaiian Islands and northeast of the Philippines, were strategically situated to serve as an air base for the Army Air Corps' long-range bombers to make non-stop strikes on nearby Japan. Because of this, American forces constructed air bases on the islands of Guam and Saipan. Japan saw the Marianas as the last line of defense for the protection of Japan and its possessions in the south Pacific.
On March 31, 1944, two planes bound for Mindanao were caught up in a tropical storm. One plane carried Admiral Mineichi Koga, who replaced Admiral Isoroku Yamamoto as commander-in-chief of the Japanese Combined Fleet after Yamamoto's death. The other carried Vice Admiral Shigeru Fukudome. Both crashed into the sea off the southern Philippines. When Koga's plane smashed into the ocean, it left no survivors. Fukudome and ten of his staff, however, survived their plane crash.
These survivors were picked up by Filipino guerrillas, but left behind was a box containing a leather-bound copy of documents bearing the letter "Z" on its red cover, as well as other documents, including a study of fleet operations and a sheet of code rules. When it later washed ashore, it was recovered by two Filipino men who, guessing at their value, quickly dried the documents and hid them, buried underground in the box.
1Greg Bradsher, researcher for the National Archives and Records Administration, writes in his historical account of the events surrounding the Z-Plan that Koga held onto the briefcase, whereas James C. McNaughton, US Army historian, writes that it was Fukudome who was entrusted with the documents. See Bradsher, "The 'Z-Plan' Story: Japan's Naval Battle Strategy Drifts into US Hands, Part 2," Prologue Magazine 37.3 (2005), accessed January 16, 2015, http://www.archives.gov/publications/prologue/2005/fall/z-plan-2.html, and McNaughton, Nisei Linguists: Japanese Americans in the Military Intelligence Service during World War II (Washington, DC: US Army Center of Military History, 2006), pp. 245-6.