The years 1909-1918 can be regarded as formative for MI5, an era in which it developed from a small counter-espionage bureau into an established security intelligence agency. MI5 had two main roles during this period; counter-espionage, and advising the War Office on how to deal with the police and the civilian population, particularly foreign nationals in Britain. MI5 began as a one-man affair in 1909, tasked with the limited remit of ascertaining the extent of German espionage in Britain amidst an uncertain future. By the armistice MI5's role had expanded considerably and it had begun to develop into an established security intelligence agency, with hundreds of personnel spread over six branches covering the investigation of espionage, records, ports and travellers and alien workers at home and overseas. This book offers an original and important contribution to our knowledge of the origins of Britain's security services. In using the example of MI5's contest against German spies during the First World War era, it forms a ground-breaking study of counter-espionage strategy and tactics, and it poses the stimulating question of'how to measure' the effectiveness of a counter-espionage agency. It also sets out probably the most detailed description of MI5's organisational structure available. MI5 at War 1909-1918: How MI5 Foiled the Spies of the Kaiser in the First World War provides a valuable and indispensable assessment of MI5 during its earliest and perhaps most challenging years.