Magic: The Codebreaking Secret Behind Pearl Harbor, Midway, and Internment

On the afternoon of December 7, 1941, Secretary of State Cordell Hull had to act surprised when Japanese diplomats delivered a message he had already read in translation. This episode explores the Allied cryptanalysis project known as Magic, from a stolen Japanese naval codebook in 1923 through the Blue system broken in 1932 to the machine cipher the Americans called Purple, which William Friedman and Frank Rowlett's team reverse engineered without ever seeing the hardware. It explains how cribs from formal diplomatic greetings let them deduce the machine's internal wiring.

The episode then looks at the dysfunctional system built around this triumph: an Army and Navy rivalry so bitter that the services alternated days handling decrypts, intercepts sometimes mailed to Washington, translation bottlenecks, and courier briefcases chained to wrists so policymakers could read but never copy. It addresses why Magic did not prevent Pearl Harbor, the intelligence windfall from the Japanese ambassador in Berlin, the JN-25 breakthroughs behind Coral Sea and Midway, the leaks that somehow never reached Tokyo, and the ongoing historical debate about whether Magic intercepts influenced Executive Order 9066 and the internment of Japanese Americans.

• Why Japanese military hardliners kept their own Foreign Office in the dark about the Pearl Harbor attack, leaving nothing operational in the diplomatic traffic the Americans were reading
• How General George Marshall's secret letter persuaded candidate Thomas Dewey to stay silent about codebreaking during the 1944 election
• The Chicago Tribune story by Stanley Johnston after Midway, the congressional debate it triggered, and the fact that over 500 people at the White House were reading Magic intercepts
• How Japan only learned Purple had been broken during post-war congressional hearings while still using the machine under American occupation
• The competing arguments about Executive Order 9066, including consulate cables about recruiting spies, the fact that General DeWitt lacked Magic clearance, and the 1988 congressional apology