|"Tell Jimmy to get on his horse."|
In 1922 the major naval nations met to negotiate limits to the sizes of navies, and Britain hoped, agree to eliminate submarines. But France and others wanted to keep submarines.
To help our negotiators know what the Japanese were planning, New York City detectives, FBI agents, and Naval intelligence agents broke into the Japanese Consulate in New York City and photographed code books. A retired missionary to Japan was hired to translate these into English. Our negotiators knew the instructions radioed to Japanese negotiators before negotiating sessions.
As the codes became more complex, the Navy was able to continue breaking them. In the 1930s the Japanese switched to a machine code, and code breakers were able to hand build a machine that read the code. The code breakers were highly skilled people, some civilian, some Navy, some Army. One of the most skilled was a remarkable genius, Mrs. Agnes Meyer Driscoll.
IBM punch card machines sorted through and organized information. At Pearl Harbor in 1942 3-million punch cards per month went through the machines to analyze the 5-digit Japanese code groups. Commander Joe Rochefort was in charge, a code genius with great memory, who knew the nuances of the Japanese language. He and his group worked almost round the clock. They were known as FRUPac — Fleet Radio Unit Pacific. Midway through the war the name was changed as it moved to larger quarters and grew to over a thousand people, but often still was called FRUPac.
The Japanese developed these codes by 1941:
In addition there was a code of the Pacific ocean to give locations that we needed. While FRUPac broke information about the upcoming Battle of Midway in the spring of 1942, locations were called AF, AK, etc. Cmdr. Rochefort and his code breakers figured out what some meant. In January 1943 the Japanese submarine I-1 was attacked by 2 corvettes near Guadalcanal. First the Kiwi blanketed the larger I-1 with gunfire, then rammed it 3 times. It sank in very shallow water. Australian and American swimmers quickly dove through it, retrieving documents and codes, including the coded grid of the Pacific. Now our crypto analysts knew the codes for all locations.
From the 1920s the Navy sent top graduates from the Naval Academy to Tokyo to serve for two years as assistant naval attaches to master the Japanese language, and many of these then went into intelligence. For example, at the Battle of the Philippine Sea, June 1944, one of these, Lieut. (jg) Charles A. Sims, translated the orders the Japanese air coordinator radioed to his bombers, so our Hellcat pilots were vectored to the best altitude and position over where the enemy bombers had been told to go ready to dive to the attack. Our pilots were put in the most effective place before Japanese bombers arrived.
Using the noon positions and other messages in the maru code, and the coded Pacific grid, the code breakers gave information by secure line to Admiral Lockwood, Commander Submarines, who with his staff vectored the most appropriate submarines to where they could expect targets. Interceptions were planned at night when submarines on the surface could use radar. At the end of the war 231 sea going Japanese merchant ships remained afloat.
Jimmy to get on his horse"
Several books tell the history of this code breaking. A good summary is in Silent Victory by Clay Blair, Jr., in his opening chapters. Much more detail is in many other books such as And I Was There by Adm. Edwin Layton.
Copyright © 2004 John F. Yeaman