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Ultra was the Allies' name for highly classified intelligence information obtained during World War Two by breaking encrypted enemy radio communications. “It was thanks to Ultra that we won the war,” Winston Churchill is reported to have told King George VI.
Much German radio traffic was encrypted on the Enigma machine, many codes of which the British could decipher. The downside was that military commanders sometimes could not act on Ultra intelligence because it might give away to the Germans that the Allies had access to Enigma traffic.
For example, during the Battle of Crete in 1941, the Allied commander on the island, New Zealand’s General Freyberg, did not move troops away from defending coastal areas despite Ultra intercepts indicating that a seaborne attack was improbable and an airborne invasion most likely. Freyberg alone had Ultra access but he did not act on it lest it reveal to the Germans that he knew their intentions. This led ultimately to the fall of Crete