Introduction

 

When one thinks of soldiers in the Pacific Theatre of World War II, they usually think of men who drove the ships, flew the planes, manned the guns, and eventually waded ashore on various islands. Yet some had an entirely different kind of contribution to the war effort that was just as, if not more, important. For example, among the most consequential were those whose job it was to intercept and decrypt Japanese radio traffic. Around 25 of these men would work around the clock in a cold and musty basement on Pearl Harbor, also nicknamed the “dungeon,” where they would parse through every single Japanese radio transmission they could get their hands on, hoping to glean some useful information.

 

One of these men was Ensign Joseph J. Rocherfort. Rocherfort was a tall, thin, soft-spoken man who never wanted or asked to be a code breaker. Yet when the commanding officer of the oiler he was stationed on was asked to nominate someone for the code and signals section, he nominated Rocherfort, as he was good at crossword puzzles. After this, Rocherfort was sent to Japan, where he would study the Japanese language and culture in preparation for the job. Once he became a commander, Rocherfort was sent to Pearl Harbor to take over the Combat Intelligence Unit, also known as Station Hypo, where he and his staff would attempt to decrypt the Japanese Navy’s operational code.

 

     Decrypting the Japanese Code

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The operational code, or JN-25, was comprised of 45,000 five-digit number groups, and each group often had filler numbers to confuse American codebreakers. For example, A standard encrypted message would look something like this: 

48933            19947             62174               02849                 62581             34875

Not only that, but the Japanese would encrypt all radio transmissions a second time using a cipher tablet. The encoder would select a five-digit number from the tablet and add it to the first number group in the message, and then the next cipher was added to the second number group, and so on and so forth. So the code group for “South” might actually be 10236, but due to being double encrypted, the encoder would add another number group from the cipher, like 45038, so it would instead show up as 55264. As you can see, the encryption process was extremely complicated, which was why the Japanese were so confident that the American codebreakers couldn't decrypt their transmissions. For the receivers of the transmission to correctly decrypt the message, they would need the code book, the cypher tablet, and then the indicator, which was hidden in the transmission, showing how to subtract the cypher from the original encryption. Eventually, the Japanese blundered, as when they updated their operational code from JN-25 to JN-25b, they sent out two identical messages: one encrypted and one not. This was because not everyone had received the new codebooks yet. The U.S. codebreakers at Station Hypo would use this as their own Rosetta Stone. Despite achieving a major breakthrough, codebreakers were still only able to decrypt a fraction of the intercepted transmissions. After a lot of trial and error, Rochefort and his team became proficient enough at decoding Japanese transmissions that they were able to translate approximately 10–15 percent of each message. For example, here is an actual transmission that Station Hypo decrypted on May 5th, 1942: 

“KAGA and (blank) (blank) less (blank) and (blank) will depart Bungo Channel (blank) May 4th and arrive (blank) (blank)” It was Rochefort and his team’s job to fill in the blanks and inform the top brass of the U.S. navy on what Japan’s plans were.

 

     What's AF?

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On May 7th, Rochefort and his team intercepted a message that revealed that the Japanese would be holding an aviation conference in which four Japanese fleet carriers would participate. Another message associated these carriers with a battleship division and a cruiser division, describing preparation for some kind of operation. Rocherfort was certain that the next offensive would be at Midway. The Japanese fleet’s reported buildup all showed signs that their next target was somewhere in the central Pacific. Furthermore, the determining factor that convinced Rochefort the target was Midway was the frequent use of the geographic designator "AF" in many of the latest Japanese transmissions. In the Japanese geographical letter system, “A” stood for American possession, and the other letter tended to describe the target in more detail. (With something like AH standing for Hawaii) Here, F stood for Airfield, as that was Midway’s primary purpose. The prospect that AF was Midway was further confirmed when a Japanese seaplane, reporting weather conditions near Midway, flew back to its base, reporting that it was near AF. 

 

However, Washington still wasn't fully convinced. The top brass was worried that the Japanese might actually be preparing an attack on either Port Moserby, New Caledonia, or Fiji instead. Incredulous that they didn't see the obvious, Rocherfort and his staff devised a scheme that would shed light on the obvious and make it impossible to deny that AF was midway. Rocherfort had a message delivered from Midway to Pearl Harbor stating that the saltwater evaporator had broken down and they were running out of fresh water. Like clockwork, two days later, an intercepted Japanese message reported that AF was short of drinking water. 

 

How Superior Intelligence Affected the Pre-Battle Phase

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The Intelligence that Station Hypo had on the Japanese operation was so good in the end that they not only knew the strength and composition of the Japanese forces, but also where the Japanese carriers would be when they launched their planes. Furthermore, with the evidence they collected, the codebreakers were even able to estimate that the date of the attack would be on June 3rd, which was remarkably only one day off from the actual date. Due to the superior intelligence the US had on Japanese plans, Admiral Nimitz was able to station hundreds of scout bombers, patrol planes, torpedo bombers, Marine fighters, as well as nineteen B-17 Flying Fortresses on Midway, all of which would play a pivotal role in the upcoming battle. Not only that, but because they knew when and where the attack was coming, the navy decided to rush stopgap emergency repairs onto the aircraft carrier USS Yorktown (which had been heavily damaged at the Battle of the Coral Sea) so that it could participate in the battle. Suppose Yorktown hadn't been present at Midway. In that case, the battle might have gone extremely poorly for the U.S., as Yorktown’s dive bombers were responsible for sinking the carrier Soryu and critically damaging the carrier Hiryu–two carriers that could have easily sunk the other 2 American carriers that were present at the battle. Most importantly, though, instead of sallying out from Pearl Harbor to Midway and springing Yamamoto’s trap, the American carriers would instead be lying in wait on the flank of the Kido Butai 325 miles north of Midway, waiting for the right time to trigger the ambush. 

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How the Disparity of Intelligence Between the U.S. and Japan led to an American Victory

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Furthermore, while many focus on the effects that superior intelligence had before the battle occurred, few discuss how the intelligence discrepancy between the two nations influenced the combat of the Battle of Midway itself. In contrast to the superb intelligence that the American forces at Midway received, Japan’s intelligence apparatus failed at multiple levels. The most crucial failure, that Japanese Air Staff Officer Yoshioka Tadakazu even admitted, was an unshakable belief among Admiral Nagumo and his staff that no U.S. carriers would be present on the morning of June 4. This assumption was so entrenched among Japanese leadership that it led to scaled-back reconnaissance efforts and an underestimation of enemy presence, despite several warning signs. For example, the staff aboard Yamato intercepted a U.S. carrier call sign near Midway on the night of June 3, yet didn't pass this intelligence on to Nagumo. Not only that, but the Japanese were supposed to carry out a reconnaissance mission called Operation K, which might have detected the American carriers, yet this was canceled at the last minute. The fact that the mission was canceled was never communicated to Admiral Nagumo, who was running all carrier operations. Because of this, Nagumo entered the battle blind, completely unprepared for a carrier battle. To make matter even worse, Yoshioka admitted that a damning sentence from a message log—“It is calculated that enemy Kidō Butai will not be encountered today”—was deliberately deleted from the postbattle report to protect the Navy’s reputation, confirming just how flawed the assumptions and mindset was of Japanese leaders before the battle. 

 

In retrospect, it is clear that one of the main reasons that the Battle of Midway was won, if not the main reason, was due to the amazing intelligence that the United States had before the battle, as well as the sheer discrepancy of intelligence quality before and during the battle between the two nations.