Counterfactual history is now a well-established means of trying to explore what might or might not have happened in the Second World War, had different decisions been taken. If only ‘x’ had been the option chosen, ‘y’ could have been the consequence, or perhaps ‘y + 1’ (the algebra, of course, is infinitely elastic). Richard Bassett – in what is otherwise a brief and readable biography of the man who headed German military intelligence from 1935 to 1944, Admiral Wilhelm Canaris – argues that, if only the British had listened to Canaris, war might have been avoided in 1939 altogether, or, once started, could have been turned into a fight against Soviet Communism with a Hitler-less Germany on the side of the Western Allies. The ‘mystery’ alluded to in the title is little more than this, and it is not difficult to understand why neither counterfactual scenario ever became historical fact.

Canaris was in many respects an unlikely candidate for the critical post of chief of military intelligence (Abwehr) in Hitler’s new Reich. From a solid bourgeois family in the Ruhr, Canaris joined the German navy in 1904, became a minor spy almost by chance after his ship was scuttled in