The brilliant work carried out by British codebreakers based at Bletchley Park is now believed to have shortened the duration of World War II by up to two years. But during the dark days of 1941, as Britain stood almost alone against the apparently unstoppable tide of the Nazi war machine, this remarkable achievement seemed a million miles away. With the help of their Polish counterparts, the British codebreakers had broken the German Enigma machine cipher. But the resources on which they could call were so small that it seemed their achievements would be in vain. Without the necessary manpower, they would never be able to take full advantage of their ability to break the German codes and ciphers. In October 1941, four of the leading codebreakers, including the outstanding Alan Turing, wrote a letter to Winston Churchill asking for help. Reminding him that on a recent visit to Bletchley Park he had praised their work, they told him that it was being held up and in some cases not being done at all, principally because we cannot get sufficient staff to deal with it. Churchill insisted that they be given everything they needed, adding the succinct instruction: action this day. It was to be a key turning point for the codebreakers, if not for the war itself. To mark the 60th anniversary of Churchill's action this day, Michael Smith and Ralph Erskine, both authorities on the work of Bletchley Park, have assembled a number of key writers to explain its importance in the history of 20th century codebreaking and the birth of today's computer age. The authors include several of those who worked at Bletchley Park, some of whom have only now agreed to tell their stories, as well as a number of prominent experts on various aspects of the codebreakers' extraordinary achievements. The contributors and editors have donated their royalties from the publication of this book to the Bletchley Park Trust, originally set up to commemorate the codebreakers' work.