A new authorised history of MI5 throws light on the dealings of the secret service.
MI5's task is described in its motto, Regnum Defende - Latin for defence of the realm.
It is a phrase which is deceptively simple, but the true meaning of which has often sparked deep controversy and suspicion.
It is also a title borrowed for the first authorised history of MI5, The Defence of the Realm: The Spooks and the Politicians, which uses the organisation's own secret records to tell its story from 1909 to the present day.
When it was created in 1909, MI5's function was simple - catching German spies in Britain.
World War II marked a high point, with MI5 taking the lead in the legendary "double cross" - turning incoming German spies to supply false information back to their controllers.
In the early Cold War, catching Soviet spies became a particularly painful task once it became clear that members of MI5, and its foreign intelligence counterpart MI6, were among those who had been handing over secrets.
It was during the Cold War that MI5's work became entangled more closely with politics, as it focused on countering "subversion" - those attempting "to undermine or overthrow parliamentary democracy by political, industrial or violent means."
