I saw this 2017 U.S. Naval Institute book at the National WW II Museum in New Orleans a week ago today, and finished it two days later. It is full of detail you wouldn't expect from a dispute that blew up in 1942 as a result of a Chicago Tribune article containing details of a secret message from CinCPac commander Chester Nimitz to the ships under his command. That it does is because the article triggered a full scale FBI and Navy investigation which produced numerous documented interviews which became accessible a number of years ago, and eventually a grand jury proceeding, the transcript of which was unsealed just two years ago.
It's for that reason that we know so much about a piece of note paper in a state room of a ship which was carrying many of the officers and men of the just-sunk carrier Lexington back to San Diego, where it docked just as news was breaking of the Navy's triumph at Midway. Living in the stateroom with the executive officer of the Lexington was Johnston, an Australian who was working as a war correspondent for the Tribune, and who was already working on the manuscript of his immensely popular book Queen of the Flattops about the Lexington's fight at the Battle of the Coral Sea.
Johnston admitted that he saw and copied the exec's list of Japanese units and ships and units, which were certainly copied from Nimitz' message, since the misspellings in the ship names of the Tribune article matched those of the message. It is also likely he heard what the ships and units referred to – the likely contents of the Japanese fleet bearing down on Midway. This was because the stateroom became a sort of after-hours hangout for the Lex's former officers, who insisted on having the ship they were hitching a ride on decode the current messages from CinCPac and circulate them. And in addition to conversation identifying what the list of ships referred to, he may have seen the original message itself, security had become so lax on the ship as it plodded towards California.
So when he heard on landing that the Japanese had just lost four carriers at Midway, he realized that the list he was literally carrying in his pocket had four carriers listed on it. One thing led to another and before the Yorktown had even sunk, the last casualty of the Midway battle, the Tribune was running a front page headline, "NAVY HAD WORD OF JAP PLAN TO STRIKE AT SEA" based in part on the note he carried.
The Navy was furious at having its codebreaking activities event hinted-at, and the Roosevelt administration – who was a frequent target of the Tribune and Navy Secretary Frank Knox – who was publisher of the Tribune's cross-town rival – were as well. The Administration pressed Attorney General Biddle to prosecute Johnston and the paper's editor under the Espionage Act – the only such prosecution of a newspaper during the entire conflict, but the prosecution fell apart in the middle of the grand jury proceeding when the Navy refused, point-blank, to provide witnesses to testify about its code breaking activities and that the article had caused damage in the form of the changing of Japanese codes. Without the necessary testimony the special prosecutor (Hoover's Attorney General, as it turned out) could not get the grand jury to indict. The Navy's wariness was well-founded – if the paper hadn't tipped the Japanese off that their code was compromised, the prosecution certainly would, if it had not already.
Events seemed to have proven the Navy right. While the Japanese regularly changed the codes during the war, including a week after the prosecution was announced, those were regularly scheduled updates, and the basic structure of the code never changed. While nothing can be said with absolute certainty, there is no record that Japan ever realized that its codes had been successfully broken. To put it simply, Japanese intelligence wasn't reading the Chicago Tribune, and also managed to miss the radio and subsequent newspaper articles regarding the grand jury proceedings.
Johnston's was not the only leak of the Japanese order of battle for Midway based on Nimitz' mid-May message, although it was the most public. According to historian Alvin Kernan, who was on the Enterprise during the battle, historians eventually traced the leak within the Pacific Fleet at least to Admiral Fletcher's sharing the message with his staff while on the Yorktown returning from Coral Sea. In fact there were numerous leaks already known in 1942, as Carlson points out, noting that they caused prosecuting attorney Mitchell headaches trying to tie any specific leak to Japan's changing their codes – the harm necessary for a prosecution under the Espionage Act. In the end it did not matter because the Navy simply would not talk about the sources of its information.
It's a well-written book about an interesting bit of the Pacific war story.