Interrogation reports and photographs are among secret MI5 files about World War One spies being made available online for the first time.
Among the spies detailed are Swallows and Amazons author Arthur Ransome and Dutch spy Mata Hari, who was executed for spying for the Germans
More than 150 files are being made available in the digitised release.
It is part of a series by the National Archives to mark the centenary of World War One.
Among the documents are also surveillance reports on organisations such as the Bolshevik Party, British Communist Party and The Boy Scout Association.
Political figures, from known fascists to communists and Russian leaders such as Leon Trotsky and Vladimir Lenin also feature.
The files also include details about American poet and author Ezra Pound, and British nurse Edith Cavell who saved soldiers in German-occupied Belgium.
The release by the National Archives, in Kew south-west London, forms part of its "First World War 100" programme which is honouring the anniversary of the war.
Records specialist, Dr Stephen Twigge, at the National Archives said: "The files in the National Archives' collection reveal the importance of the security service in safeguarding the nation during the First World War.
"Now that we have made the files available online as part of our First World War 100 programme, people across the globe can discover the secret history behind the war for themselves."