Pearl Harbor - "Magic"
by Gregory Karpicky
The Americans had not been idle during the build up to the Pacific conflict. As
early as the twenties, American and British cryptanalysts had forged an
agreement. As the British had a lead in deciphering Japanese naval codes, they
would concentrate their efforts on those while the Americans would work on
By 1939 the Americans had finally broken the Japanese diplomatic code that
would eventually become known as "Purple". "Purple" enabled President Roosevelt
and his staff to be privy to Foreign Minister Oshima's communications to
Ambassador Nomura. This gave the U.S. an invaluable advantage in reading
diplomatic messages. But that advantage did not translate into an advantage in
naval intelligence or intentions in the Pacific. The Japanese militarists, by
this time fully in charge of the Japanese government, were dead set
on a war with Britain and the U.S. and did not want their embassies to know. So
while Foreign Minister Oshima may have known more than he let on, he certainly
did not communicate that to his ambassadors.
By the fall of 1941 the English had finally deciphered about one tenth of JN25.
But that allowed a lot of information to slip through the cracks. Also the
Japanese at that point were changing ciphers about every ten days and being
very careful with military radio traffic. The Japanese navy had also recently
changed call signs on all their ships and only about 20% of the fleet had been
identified by December 7th. However, they then reassigned individual radio
operators, so Allied intelligence would recognize their "fists" and believe the
ship's location: i.e., that ships they were assigned to were in southern
Japan and not in the northern Pacific.
On November 29th the famous "Winds" message was sent, warning that conflict
with Britain, the United States, or the Soviet Union may break out soon. If any
eventuality were to take place and normal communications could not be used, a
simple, coded message, disguised as a weather report, would be broadcast over
"In case of danger with Japan-U.S. relations, the words: higashi no kaze, ame
(East wind, rain)
Japan-Soviet relations, the words: kita no kaze, kumori (North wind, cloudy)
Japan-British relations, including invasion of Thailand, the words: nishi no
kaze, hare (Westerly wind, fine.)
This signal will be given in the middle and at the end as a weather forecast
and each sentence will be repeated twice. When this is heard please destroy all
code papers, etc." 
While most sources say that the message to initiate action was never received,
there are at least two that say it was. One was a Michael Smith, who said that
the "execute" was received late in December 7th Hong Kong time, (early December
7th West Coast time.) The other was an unnamed sergeant who never testified in
any of the formal investigations. Even had the message been received and
distributed, it would at most have given only a few hours to prepare.
The battleships at Pearl would take that long just to raise steam.
But no one was thinking only of an attack on Pearl. By early November the
situation in the Pacific was grave enough for General George C. Marshall and
Admiral "Betty" Stark to send a "war warning"  to the various
commanders in the Philippines, at the U.S. Panama Canal Zone, and in
Pearl Harbor. But the intelligence available indicated the Japanese were
sailing south towards Malaya and Thailand. Conventional thinking at the time
said that, if war broke out, the Japanese would secure their flank by
attacking the Philippines as well, which they did. But there were no decrypted
messages in "Purple" or JN25 that would lead either England or America to
believe that Hawaii was the main target. By the time the Japanese had decided
on war, American and British leaders were aware of an impending conflict, but
did not know where it would begin.
. Hearings before the Joint Committee on the Investigation of the Pearl
Harbor attack, Congress o f the United States, Seventy-ninth Congress,
. At Dawn We Slept; Gordon W. Prange, p. 406.
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Copyright © 2005 Gregory Karpicky.
Written by Gregory Karpicky. If you have questions or comments on
this article, please contact Gregory Karpicky at:
About the author:
My literary background has mostly been in writing for various businesses I have consulted for. My father was a career Army officer who earned the Legion of Merit, and the exposure I received at various postings around the world with this gave me a lifelong interest military history.
Published online: 12/17/2005.