MI5 used a method that depended on strict control of entry and exit to the country and, crucially, large-scale inspection of mail.
In post-war years, attention turned to attempts by the Soviet Union and the Comintern to surreptitiously support revolutionary activities within Britain.
Information held by the service is exempt from disclosure under section 23 of the Freedom of Information Act 2000.
The Security Service is derived from the Secret Service Bureau, founded in 1909 and concentrating originally on the activities of the Imperial German government as a joint initiative of the Admiralty and the War Office.
The service has had a national headquarters at Thames House on Millbank in London since 1995, drawing together personnel from a number of locations into a single HQ facility.
Thames House was, until March 2013, shared with the Northern Ireland Office (NIO) and is also home to the Joint Terrorism Analysis Centre, a subordinate organisation to the Security Service.
Its role was originally quite restricted; existing purely to ensure national security through counter-espionage.
Operations of the service are required to be proportionate and compliant with British legislation including the Regulation of Investigatory Powers Act 2000, the Data Protection Act 1998, and various other items of legislation.MI5's expertise, combined with the early incompetence of the Soviets, meant the bureau was successful once more in correctly identifying and closely monitoring these activities.However, in the meantime, MI5's role had been substantially enlarged.It was consistently successful throughout the rest of the 1910s and 1920s in its core counter-espionage role.Throughout World War I, Germany continued trying to infiltrate Britain but MI5 was able to identify most, if not all, of the agents dispatched.