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Of prime importance to both the allies-to-be and the Japanese was the Japanese naval code, JN-25, in which operational messages were sent. This code was introduced in May 1939. Eric Nave (Nave, Eric) of FECB is said to have broken the code by the end of that year giving the British the ability to read Japanese messages and the American Naval Communications Service unit OP-20-G, headed by Laurence F. Stafford had some 2,000 values identified by January 1941. It was in this code that instructions were sent by Admiral Isoroku Yamamoto (Yamamoto, Isoroku) to the commander of the task force headed for Pearl Harbor, including the "Climb Mount Niitaka" message. To what extent the JN-25 messages prior to the attack had been read by OP-20-G or FECB is not known for certain, nor can it be said precisely why those messages that were decoded were not acted on.

See also: CIU; Climb Mount Niitaka; espionage; FECB; Nave, Eric; Pearl Harbor; Yamamoto, Isoroku