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’.
Why MI5 was founded; the fight against German espionage; how MI5 caught German spy Carl Hans Lody.
During World War II, MI5 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.
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.
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.