In 1921 MI5 commissioned a detailed review of the organisation's operations during the First World War. The survey, amounting to over 270,000 words in seven volumes, plus appendices, remained classified in MI5's registry and was only used in Whitehall to argue against budget cuts. MI5 in the Great War is a version of that top-secret history, concentrating on the main narrative of the Secret Intelligence Service's activities in the war, edited and brought up to date by bestselling writer and historian Nigel West. Recently declassified and published here for the first time, this fascinating document was never intended for circulation outside of government, and is of considerable historical significance. The topics covered include a detailed account of German pre-war espionage, and the extent to which the Kaiser's own spies developed an extensive network in Britain; previously undisclosed details from the individual MI5 dossiers compiled on enemy spies executed in England, including that of Sir Roger Casement, who was famously hanged for treason; MI5's abortive attempt to manage a double agent reporting to the German Nachrichtenstelle in Rotterdam, and the detection of attempts made by the enemy to infiltrate MI5 with double agents.