Battle of Midway History book

Admiral Nimitz had one critical advantage: U.S. cryptanalysts had partially broken the Japanese Navy's JN-25b code.[51] Since early 1942, the U.S. had been decoding messages stating that there would soon be an operation at objective "AF". It was initially not known where "AF" was, but Commander Joseph Rochefort and his team at Station HYPO were able to confirm that it was Midway: Captain Wilfred Holmes devised a ruse of telling the base at Midway (by secure undersea cable) to broadcast an uncoded radio message stating that Midway's water purification system had broken down.[52] Within 24 hours, the code breakers picked up a Japanese message that "AF was short on water".[53] No Japanese radio operators who intercepted the message seemed concerned that the Americans were broadcasting uncoded that a major naval installation close to the Japanese threat ring was having a water shortage, which could have tipped off Japanese intelligence officers that it was a deliberate attempt at deception.[54] HYPO was also able to determine the date of the attack as either 4 or 5 June, and to provide Nimitz with a complete IJN order of battle.[55]