In pre-war Britain, the growing military threat of Germany created a climate in which popular novels about espionage thrived. Writers such as Erskine Childers, author of The Riddle of the Sands (1903), and William Le Queux depicted a sophisticated German intelligence network laying the foundations for an invasion of Britain.

The truth was more prosaic. A small number of spies employed by the German navy were active in pre-war Britain. But between August 1911 and July 1914, the War Office's counter-espionage department (known today as MI5) arrested just 10 suspects. Britain's own attempts to establish a spy network in Germany met with similarly little success.

The triumph - even in the highest government circles - of journalistic fantasy over mundane reality had immediate repercussions when war broke out in August 1914. An unprecedented 'spy mania' gripped Britain. Although 21 real German spies were arrested on 4 August, thousands of imaginary acts of espionage were reported to credulous police and military authorities.

Homing pigeons, seen to be capable of carrying messages to the enemy, were killed. Thousands of false accounts of suspicious 'night-signalling', by which German spies might guide Zeppelins or submarines towards their targets, were submitted. Le Queux's German Spies in England: An exposure, published to favourable reviews in February 1915, fabricated a system of German espionage that ranged from German prostitutes around Piccadilly Circus to 'naturalised' businessmen of the highest social standing. Such claims inevitably encouraged anti-German sentiments. On 13 May 1915, the Asquith government decided to intern all 'enemy aliens' residing in Britain for the duration of the war.

Similar, usually unfounded suspicions about German espionage activity in Britain were raised throughout the war. Enemy agents were variously accused of infecting cavalry horses with anthrax, starting fires in ports and posing as circus performers or commercial travellers to gain intelligence information.

In reality, the wartime operations of German espionage in Britain under Gustav Steinhauer were limited and largely unsuccessful. Between August 1914 and September 1917, only 31 German spies were arrested on British soil, 19 of whom were sentenced to death and a further 10 imprisoned. Enemy spy activity thereafter was so negligible that no further espionage trials took place during the war.

Many of the men recruited by the Germans for intelligence operations were untrained and inept amateurs. Witness the case of Carl Hans Lody, the first wartime German spy to be executed in Britain, in November 1914. Viewed even by his captors as a decent and patriotic man, Lody left a trail of clues from his intelligence-gathering operations in Britain during August and September and was arrested in Ireland on 2 October. Most of the information that he sent to his superiors - such as the telegram on 4 September describing how Russian troops had apparently been seen marching from Aberdeen to the south of England - was useless.

Magdeburg

Room 40

Even on the Western Front, where War Office intelligence operations did not always run smoothly, there were great successes, most notably the use of homing pigeons to carry messages to and from operatives working behind enemy lines. Espionage thus played a small but significant role in the eventual Allied victory over Germany and its allies in 1918.