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US codebreaking vs. Japan (Magic, Purple, JN-25)
Purple broken September 1940; JN-25 read before Midway, 1942· Event· Technology, codes & intelligence
Well documentedSolid records, physical evidence or scientific consensus.
American codebreaking against Japan: 'Magic' read diplomatic cables in the Purple cipher, and Navy teams partly read the fleet code JN-25.
See it on the Battles & Wars map →What happened
In September 1940 the Army's Signal Intelligence Service, under William Friedman and Frank Rowlett, broke Purple, Japan's diplomatic cipher machine, and built copies without ever seeing one. Its decrypts were called Magic. Navy codebreakers, including Joseph Rochefort's Station Hypo at Pearl Harbor, attacked JN-25, the main naval code. By May 1942 they read enough to predict Japan's attack on Midway. In April 1943 a decoded flight schedule let US fighters shoot down Admiral Yamamoto's plane.
Why it matters
Codebreaking let a weaker US Pacific Fleet ambush Japan's carriers at Midway, the turning point of the Pacific war.
How we know
Declassified NSA and Navy histories, the 1945–46 congressional Pearl Harbor hearings that made Magic public, and memoirs such as intelligence officer Edwin Layton's And I Was There (1985).
Still debated
Some still claim Washington ignored or withheld warnings before Pearl Harbor; most historians find failures of analysis and sharing, not a plot.
Connections (4)
▸US codebreaking vs. Japan (Magic, Purple, JN-25) revealed the Japanese plan for the Battle of MidwayWell documented
The connection
In spring 1942 Station Hypo at Pearl Harbor read parts of Japanese naval messages pointing to a target coded 'AF'. To confirm it was Midway, the island's base sent a fake uncoded report that its water plant had failed; Japan soon reported 'AF' short of water. Admiral Nimitz placed his three carriers in ambush.
When and how
Pearl Harbor and Washington, May 1942; Commander Joseph Rochefort, fleet intelligence officer Edwin Layton, and Admiral Chester Nimitz.
How we know
Declassified NSA and Navy records and the decrypts themselves document the intelligence picture and the water ruse.
Debates and open questions
Washington's codebreakers first doubted Hawaii's reading, and Rochefort was later sidelined; how credit should be shared is still argued.
▸US codebreaking vs. Japan (Magic, Purple, JN-25) led to the 1943 shootdown of Isoroku YamamotoWell documented
The connection
In April 1943 US codebreakers read a message giving Yamamoto's exact schedule for an inspection flight near Bougainville in the Solomon Islands. Long-range P-38 Lightning fighters from Guadalcanal intercepted his bomber on April 18 and shot it down, killing him. The US hid the role of codebreaking to protect the secret.
When and how
Bougainville, Solomon Islands, April 18, 1943; P-38s of the US 339th Fighter Squadron in Operation Vengeance, approved by Admiral Nimitz.
How we know
The decoded message and mission records are declassified; the wreck of Yamamoto's plane still lies in the Bougainville jungle.
Debates and open questions
Which American pilot fired the fatal shots, Rex Barber or Thomas Lanphier, was argued for decades; most evidence now favors Barber.
▸US codebreaking vs. Japan (Magic, Purple, JN-25) read Tokyo's cables yet failed to foresee the Attack on Pearl HarborWell documented
The connection
Magic let Washington read Tokyo's diplomatic cables, which showed negotiations nearing a deadline and war likely. But the cables never named Pearl Harbor, the strike fleet kept radio silence, and naval codes were not yet read well. Warnings to Hawaii were vague and late; General Marshall's final alert arrived after the bombs fell.
When and how
Washington and Hawaii, autumn 1941; the '14-part message', decoded December 6–7, 1941, and Marshall's delayed warning, sent by commercial telegraph.
How we know
Several official investigations, including the 1945–46 congressional hearings, examined what Magic showed and who saw it.
Debates and open questions
Whether the September 1941 'bomb plot' cable, which asked Japan's Honolulu consulate to grid Pearl Harbor and report ship berths, should have raised alarm is still argued; Washington read it as routine spying.
▸US codebreaking vs. Japan (Magic, Purple, JN-25) shared its secrets with Bletchley Park and Ultra codebreakingWell documented
The connection
In February 1941, before the US was at war, an American team brought a copy of the Purple machine to Bletchley Park, and Britain shared some Enigma methods in return. The 1943 BRUSA agreement made the cooperation formal, with Americans working at Bletchley and the two sides dividing the work. It grew into today's 'Five Eyes' intelligence alliance.
When and how
Bletchley Park, February 1941 (the Sinkov–Rosen mission); BRUSA agreement, May 1943; UKUSA agreement, 1946.
How we know
Declassified NSA and British records, including the UKUSA agreement papers released in 2010.
Debates and open questions
How fully each side shared, especially Britain's early reluctance to hand over Bombe details, caused friction then and is still discussed.