Established in 1909 as the Secret Service Bureau, MI5 and MI6 soon grew from modest beginnings to become professional and effective intelligence agencies. MI5, founded by Captain (later Major General) Vernon Kell, played a central role in the capture of most of Imperial Germany's intelligence agents in the UK at the start of World War I.
The establishment of the Secret Service Bureau
How and why MI5 was established, and the role of Vernon Kell in founding the Service
MI5 in World War I
The round-up of the German spy network and MI5's wartime expansion
Carl Hans Lody
The story of how MI5 captured a German spy in Britain in 1914