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 years immediately after the First World War saw MI5's size being reduced drastically as a result of post-war cost-cutting. The Service's very existence came under threat for a time, but by the start of the 1930s it had gained new importance in countering communist and fascist subversion in the UK. It also played a leading role in countering Soviet espionage and obtaining information on Nazi Germany.
During World War II, the Security Service played a key role in combating enemy espionage, intercepting German communications and feeding misinformation back to Germany.
The Second World War ended with Europe divided between the two superpowers, the United States and the Soviet Union. Subversion and Soviet espionage were key concerns during the Cold War, and from the 1970s onwards terrorism also emerged as a serious threat to national security.
As the Cold War came to an end, terrorist threats from Northern Ireland and states such as Colonel Qadhafi's Libya became priorities for MI5. Major reforms were put in place and the Service gained its first female Director General. The rise of Islamist terrorism at the end of the 1990s, culminating in the 9/11 attacks in 2001, led to major changes in the way MI5 operated.
You can learn more about this history of MI5 at The National Archives (TNA) in Kew. Since April 2000 we have released over 5,000 files for TNA visitors to examine.
"The Defence of the Realm" by Cambridge University historian Christopher Andrew is the first authorised history of MI5.
MI5 has had a variety of names since it was established in 1909. This has often led to confusion about what the Service has been called at various points in its history.