31. What Is “AF”?
American cryptanalysts had cracked Japanese ciphers, and knew that they planned to attack a target codenamed “AF”. However, nobody was sure just what AF was. So Navy cryptanalyst Joseph Rochefort devised a ruse to trick the Japanese into showing their hand.
Suspecting that AF was Midway Island, Rochefort directed Midway’s radio station to send an uncoded message, stating that its water purification system had broken down, and that the island was running out of drinking water. 24 hours later, American codebreakers intercepted a Japanese message to the effect that “AF” was running out of water. With the identity of AF thus confirmed, Chester Nimitz, commander of the Pacific Theater, sent his carriers to the vicinity of Midway, to wait in ambush for the Japanese.