In May 1942, U.S. and Australian naval and air forces were facing off against the Imperial Japanese Navy in the Battle of the Coral Sea in the South Pacific. But in a windowless basement at Pearl Harbor, a group of U.S. Navy codebreakers had intercepted Japanese radio messages suggesting Japan was planning an entirely different—and potentially far more damaging—operation in the Pacific theater.

Led by Lieutenant Commander Joseph Rochefort, the team of cryptanalysts and linguists made up the U.S. Navy’s Combat Intelligence Unit (better known as Station Hypo). By April 1942, they had gotten so good at breaking Japan’s main operational code, which they dubbed JN-25b, that they were able to intercept, decrypt and translate parts of Japan’s radio messages within hours of when they were sent.

'AF' Identified as Code for Midway

The radio traffic they intercepted that May suggested that Admiral Isoroku Yamamoto, the mastermind behind the Pearl Harbor attack, was preparing a major invasion, involving four Japanese aircraft carriers along with many other ships, at a location designated with the initials “AF.”

Station Hypo had little doubt as to what “AF” referred to: the U.S. naval and air base on Midway Atoll, two tiny islands located in the central Pacific, around 1,200 miles northwest of Pearl Harbor. Back in March, a Japanese plane reporting weather conditions near the islands had also mentioned “AF,” suggesting strongly that the designator referred to Midway.