Posted on Dec 19, 2022
The Sailor Who Discovered ADM Yamomoto’s Plans to Attack Midway
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While working on garbled JN-25 coded messages at Fleet Radio Unit Melbourne (FRUMEL), YN2 William H. Trimblay recognized the code group “ATTACK AF.”
FRUMEL immediately forwarded the discovery to Station HYPO in Hawaii and OP-20G in Washington DC. The United States was now aware of the first part of Admiral Isoroku Yamomoto’s Battle Plan in the Pacific. The BIG question, however, was where and when will the attack take place!
The attack location and time were confirmed when the American base at Midway Island sent out a false message, deliberately using a low-grade encryption system, stating U.S. forces were short of fresh water and to send a water barge. As planned, the Japanese intercepted the message and reported to their chain of command that “AF” was short of fresh water. Navy radio intercept operators intercepted and broke the Japanese message, confirming Midway Island was the location of the attack.
FRUMEL immediately forwarded the discovery to Station HYPO in Hawaii and OP-20G in Washington DC. The United States was now aware of the first part of Admiral Isoroku Yamomoto’s Battle Plan in the Pacific. The BIG question, however, was where and when will the attack take place!
The attack location and time were confirmed when the American base at Midway Island sent out a false message, deliberately using a low-grade encryption system, stating U.S. forces were short of fresh water and to send a water barge. As planned, the Japanese intercepted the message and reported to their chain of command that “AF” was short of fresh water. Navy radio intercept operators intercepted and broke the Japanese message, confirming Midway Island was the location of the attack.
The Sailor Who Discovered ADM Yamomoto’s Plans to Attack Midway
Posted from stationhypo.comPosted in these groups: 
Cryptology
WWII World War Two
Cryptology WWII World War Two
Posted >1 y ago
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PO1 William "Chip" Nagel
..."The attack location and time were confirmed when the American base at Midway Island sent out a false message, deliberately using a low-grade encryption system, stating U.S. forces were short of fresh water and to send a water barge. As planned, the Japanese intercepted the message and reported to their chain of command that “AF” was short of fresh water. Navy radio intercept operators intercepted and broke the Japanese message, confirming Midway Island was the location of the attack.
The cryptanalysts at Station Hypo Hawaii were also able to give the date of the attack (June 4 or 5) and provide the order of battle of the Imperial Japanese Navy.
Shortly before YN2 Tremblay’s discovery of AF, he along with 17 other cryptologists stationed at Corregidor, Philippines were evacuated onboard USS Permit (SS 178) on March 16, 1942, arriving in Melbourne, Australia on April 19. This was the second group to evacuate due to the Japanese landing on Bataan. The first and third group was evacuated by the USS Seadragon (SS 194). All three groups relocated to Melbourne and remained there for the remainder of the war intercepting IJN communications and reporting on their operations and movements throughout the Pacific.
YN2 Tremblay served in the Navy from 1939 to 1966, retiring as a lieutenant. He died on March 10, 2002. The significance of YN2 Tremblay’s discovery cannot be overstated!"...
..."The attack location and time were confirmed when the American base at Midway Island sent out a false message, deliberately using a low-grade encryption system, stating U.S. forces were short of fresh water and to send a water barge. As planned, the Japanese intercepted the message and reported to their chain of command that “AF” was short of fresh water. Navy radio intercept operators intercepted and broke the Japanese message, confirming Midway Island was the location of the attack.
The cryptanalysts at Station Hypo Hawaii were also able to give the date of the attack (June 4 or 5) and provide the order of battle of the Imperial Japanese Navy.
Shortly before YN2 Tremblay’s discovery of AF, he along with 17 other cryptologists stationed at Corregidor, Philippines were evacuated onboard USS Permit (SS 178) on March 16, 1942, arriving in Melbourne, Australia on April 19. This was the second group to evacuate due to the Japanese landing on Bataan. The first and third group was evacuated by the USS Seadragon (SS 194). All three groups relocated to Melbourne and remained there for the remainder of the war intercepting IJN communications and reporting on their operations and movements throughout the Pacific.
YN2 Tremblay served in the Navy from 1939 to 1966, retiring as a lieutenant. He died on March 10, 2002. The significance of YN2 Tremblay’s discovery cannot be overstated!"...
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I was not aware 17 cryptologists working on Japanese code had evacuated the island Fortress of Corregidor in the Philippines a month before.
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PO1 William "Chip" Nagel good day Brother William, always informational and of the most interesting. Thanks for sharing, have a blessed day!
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