Churchill used secret intelligence on a global scale, freely shared it with the Americans, and made it count in the Battle of the Atlantic. The Cabinet's unanimous decision to aid Greece in 1941 would not have been made were it not for the Enigma decrypts.

Churchill became Prime Minister on 10 May 1940. Twelve days later, on 22 May, the codebreakers at Bletchley Park broke the Enigma key most frequently used by the German Air Force. This was the hourly two-way top-secret radio traffic between the combined German Army, Navy and Air Force headquarters at Zossen and the commanders-in-chief on the battlefronts.

Included in the newly broken key were the top-secret messages of German Air Force liaison officers with the German Army. The daily instructions of these liaison officers included targets, supply and, crucially, details of shortages such as aviation fuel. [...]

As Prime Minister and Minister of Defence, Churchill was intensely concerned with maintaining the secrecy of all aspects of war policy and planning. In no area was secrecy more important to him than with regard to Enigma. [...]

Churchill made his first visit to Bletchley on 6 September 1941. His principal Private Secretary, John Martin, who accompanied him in the car on their way to Oxfordshire for the weekend, did not enter the building, and had no idea what went on there.

Following his visit to Bletchley, Churchill received a letter, dated 21 October 1941, from four Bletchley cryptographers, Gordon Welchman, Stuart Milner-Barry, Alan Turing and Hugh O'D. Alexander. In their letter, they urged Churchill to authorize greater funding for the work they were doing. Manual decoding was extremely time-consuming. Turing believed that a machine he had devised - the “bombe,” then in its early days - could speed up the task considerably but that more funding and more staff were needed.