Security Service

The British domestic security agency, popularly known as MI5

Overview

MI5 was founded in 1909 to counter German espionage and has worked against subversion, sabotage, and terrorism ever since. It operates only within the United Kingdom and holds no arrest powers, working through cooperation with police. Its wartime Double-Cross System turned every captured German agent into a channel of deception, an achievement that helped secure the D-Day surprise. Cold War priorities were Soviet bloc operations and later Irish and Islamist terrorism. The service gained a statutory basis in 1989 and moved into its famous Thames House headquarters.