During World War II Turing helped (with Gordon Welchman) to decipher the Nazi code, using the Colossus machine designed by Tommy Flowers, building on earlier work by Polish mathematicians Marian Rejewski, Jerzy Rozycki and Henryk Zygalski who first broke the Enigma code. Some sources say this work was decisive for defeating the Third Reich.

Turing's war work counts for more than computers

Sir - John L. Casti, in his fine review of Alan Turing: Life and Legacy of a Great Thinker, edited by Christof Teuscher ("Touring artificial minds" Nature 428, 258; 2004), proposes that Turing had more impact on everyday life than the man named by Time magazine as Person of the Century, Albert Einstein (Time 154, 27; 1999). Casti suggests that Turing's 1936 paper provided the "theoretical backbone" for all computers to come.

Although Turing, a hero of mine, certainly was one of the greatest, we should keep in mind that his paper essentially just elegantly rephrased Kurt Gödel's 1931 results and Alonzo Church's extension thereof. It did not have any impact on the construction of the first working program-controlled computer. That was made in Berlin by Konrad Zuse in 1935-1941 and was driven by practical considerations, not theoretical ones.

In fact, the greatest impact that Alan Turing made on daily life was probably through his contribution to cracking the Enigma code, used by the German military during the Second World War, which is sometimes cited as a decisive event of the war.