Between the wars
The ’20s and ’30s
The years immediately after the World War 1 saw MI5’s size reduced drastically as a result of post-war cost-cutting.
Our organisation’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.
The Soviet threat between the wars
MI5 unmask a spy ring but Soviet intelligence have already begun a new recruitment strategy which would culminate in the ‘Cambridge Five’.
Find out more about the Soviet threat
After World War I MI5’s size was reduced drastically as a result of post-war cost-cutting and for a time the Service’s very existence came under threat.
During World War II, MI5 played a key role in combating enemy espionage, intercepting German communications and feeding misinformation back to Germany.
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, 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 led to major changes in the way MI5 operated.
National Archives
You can learn more about the history of MI5 at The National Archives. We periodically release files to The National Archives which cover many aspects of our work.