Did Signals Intelligence predict the attack on Pearl Harbor?

Last Updated: 07 Dec 2016
Topics: Our history
GCHQ’s Historian examines the myth that Winston Churchill kept knowledge of the 1941 attack secret.

One of the enduring myths about Signals Intelligence in the Second World War is that Britain’s Prime Minister Winston Churchill knew from intercepted messages that the Japanese were going to attack Pearl Harbor in December 1941 but kept the fact secret to bring the USA into the war on the Allied side.  A variant of the myth has US President Roosevelt as part of the secret, looking for a pretext to bring his country into the war.

The myth is false.  The strategic situation in the Far East was such that war with Japan had become more likely.  Rumours of a Japanese attack in South East Asia had been reported from Bletchley Park some days before the actual attacks, and, ironically, the report had been shared with US intelligence,  but the report was vague and assumed that the Japanese would invade Thailand.


Black and white
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Both the US and the UK had underestimated Japanese intentions and capabilities and did not imagine that they might be capable of such strategic surprise.


B&W photo showing a dramatic explosion of the ship; thick and fiery smoke clouds with projectiles
©Public Domain - National Archives and Records Administration #80-G-71198


Furthermore, the simultaneous Japanese attacks on Hong Kong, Malaya, the Dutch East Indies and Pearl Harbor were carried out in radio silence: there were no transmissions for Bletchley Park to intercept. 


B&W photo of a stricken ship billowing black smoke
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