The Japanese Navy in May of 1942 was still largely unbloodied. And it was not because they had simply avoided battle. Rather, they had pretty much stomped any enemy that had dared oppose them. Even the “setback” in the Coral Sea couldn’t really be looked on as a defeat. After all, while the Japanese had lost a carrier and some airplanes, they were still the overwhelming power in the Pacific. And the Americans had lost the USS Lexington, leaving them with just three aircraft carriers (a fourth, the USS Saratoga, had just finished repairs but was in San Diego being re-outfitted).
And now the focus was on Island AF. You wondering where that is? Island AF is actually Midway Island in Japanese code-speak. The Japanese used an assortment of coding systems for their various organizations, but JN-25 was the one used by the Imperial Japanese Navy. It was the most important and also the most secure of all Japanese ciphers. And therefore, it was the biggest target of U.S. codebreakers.
And U.S. codebreakers had cracked much of it. In fact, “Island AF” was part of the U.S. scheme as well. The messages deciphered by U.S. intelligence mentioned “Island AF” on numerous occasions, and it was suspected the reference was to Midway. So the U.S. Navy phoned the guys on Midway and told them to send an uncoded message back stating they were low on fresh water. It was done and, not long after, the U.S. intercepted and deciphered a Japanese message about “Island AF” being short of water. The U.S. had their answer. From this they learned that Midway was the subject of intense Japanese interest.
But the Japanese were also aware that their codes could be compromised, so they periodically changed them. They had done so in the weeks leading up to attack on Pearl Harbor, which greatly assisted in the success of that operation. And now, six months had passed and another major operation (Midway) was looming. It was time for another change.
But it’s possible that Japanese success over the first half of 1942 bred a little over-confidence in the Navy. On May 25, 1942, the Japanese significantly changed their JN-25 coding structure and ciphers, which meant U.S. codebreakers had to start their jobs all over again. But the change occurred right after they had broadcast their full operating plan for the attacks on Midway. And so the plans (or at least parts of it) fell right into the hands of the U.S. Navy.
For the Japanese, the change from JN-25b to JN-25c meant they would achieve surprise when they occupied Guadalcanal in July of 1942. But their attempt to take Midway would end in disaster.
Recommended Reading: Incredible Victory – The Battle of Midway