History: The Inter-War Period

Between the two World Wars, MI5 faced the increasing challenge of espionage from the Soviet Union and Nazi Germany, as well as the internal threat of subversion by communists and fascists.

Despite this, its staff numbers shrank greatly from their wartime peak of 844 staff to only about a dozen by 1929. Its founder, Sir Vernon Kell, remained in charge of the organisation throughout the inter-war period.

MI5 was renamed the "Defence Security Service" in 1929. Two years later, in 1931, it became the Security Service - the title that it still uses today. (See What's in a Name? for more on the Service's various names.)

Also in 1931, the Service gained formal responsibility for assessing all threats to the security of the UK, apart from those posed by Irish terrorists and anarchists, which remained the responsibility of the police.

The following pages provide some background information on the issues and threats that the Service faced during this period, as well as some notable cases in which we were involved.

• The Communist threat
  Soviet espionage and the rise of the Communist Party of Great Britain.


• Fascism and Nazism
  The Security Service and the threat from the far right.