To mark the centenary of its foundation, the British Security Service, MI5, has opened its archives for the first time to an independent historian. In The Defence of the Realm, Christopher Andrew reveals the precise role of the Security Service in 20th-century British history, from its founding by Captain Kell of the British Army in October 1909 through two world wars and up to and including its present roles in counter-espionage and counter-terrorism. Full of dry humour, this fascinating and thoroughly engaging book describes how MI5 has been managed, its relationship with the government, and where it has triumphed and where it has failed. Readers will also discover the identities of previously unknown enemies of Britain and the West, whose activities the Service has brought to light. Above all, they'll understand the distinctive ethos and place of this hitherto extremely secretive organization within the U.K.