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.
The inter-war years
MI5's fight for survival after World War I and its penetration of Nazi Germany's London embassy
The Soviet threat between the wars
How MI5 countered Soviet intelligence activities in the UK