The intelligence gained from cracking the German Enigma code was given the highest classification: 'Ultra.' This information was so sensitive that the Allies often allowed German attacks to happen to prevent the Nazis from realizing their code had been broken. For example, some historians believe Churchill knew about the 1940 bombing of Coventry in advance but chose not to evacuate the city to protect the Ultra secret. While this remains debated, it highlights the brutal moral calculus of wartime intelligence. Decrypts from Bletchley Park are estimated to have shortened the war by two years.