Editing MI5 (section)

===Inter-war period===
MI5 proved consistently successful throughout the rest of the 1910s and 1920s in its core counter-espionage role.  Throughout the First World War, Germany continually attempted to infiltrate Britain, but MI5 was able to identify most, if not all, of the agents dispatched. MI5 used a method that depended on strict control of entry and exit to the country and, crucially, large-scale inspection of mail. In post-war years, attention turned to attempts by the [[Soviet Union]] and the [[Comintern]] to surreptitiously support revolutionary activities within Britain. MI5's expertise, combined with the early incompetence of the Soviets, meant the bureau was successful in correctly identifying and closely monitoring these activities.<ref>{{Cite news|url= https://www.Telegraph.co.uk/history/10755313/How-MI5-combated-Communist-attempts-to-take-over-the-scouts.html|archive-url= https://web.archive.org/web/20140410114821/http://www.telegraph.co.uk/history/10755313/How-MI5-combated-Communist-attempts-to-take-over-the-scouts.html|url-status= dead|archive-date= 10 April 2014|title= How MI5 combated Communist attempts to take over the scouts|website= www.Telegraph.co.uk|publisher= The Telegraph|date= 10 April 2014|access-date=21 November 2018}}</ref>

In the meantime, MI5's role had grown substantially. Due to the spy hysteria, MI5 had formed with far more resources than it actually needed to track down German spies.  As is common within governmental bureaucracies, this caused the service to expand its role to use its spare resources. MI5 acquired many additional responsibilities during the war.  Most significantly, its strict counter-espionage role blurred considerably. It acquired a much more political role, involving the surveillance not merely of foreign agents, but also of [[Pacifism|pacifist]] and anti-[[conscription]] organisations, and of [[Trade unionism|organised labour]].  This was justified by citing the common belief that foreign influence was at the root of these organisations. Thus, by the end of the First World War, MI5 was a fully-fledged investigating force (although it never had powers of arrest), in addition to being a counter-espionage agency. The expansion of this role continued after a brief post-war power struggle with the head of the [[Special Branch]], Sir [[Basil Thomson]].<ref>{{Cite web|url= http://www.Spartacus.schoolnet.co.uk/SSthomson.htm|title=Basil Thomson|website= www.Spartacus.schoolnet.co.uk|publisher=[[Spartacus Educational]]|access-date=1 July 2012|url-status= dead |archive-url= https://web.archive.org/web/20120520021351/http://www.spartacus.schoolnet.co.uk/SSthomson.htm|archive-date= 20 May 2012}}</ref>

After the First World War, budget-conscious politicians regarded Kell's department as unnecessary.  In 1919, MI5's budget was slashed from £100,000 to just £35,000, and its establishment from over 800 officers to a mere 12. At the same time, Sir [[Basil Thomson]] of Special Branch was appointed Director of Home Intelligence, in supreme command of all domestic counter-insurgency and counter-intelligence investigations.  Consequently, as official MI5 historian [[Christopher Andrew (historian)|Christopher Andrew]] has noted in his official history ''Defence of the Realm'' (2010), MI5 had no clearly defined role in the [[Anglo-Irish War]] of 1919–1921. To further worsen the situation, several of Kell's officers defected to Thomson's new agency, the Home Intelligence Directorate.  MI5 therefore undertook no tangible intelligence operations of consequence during the [[Irish War of Independence]].  MI5 did undertake the training of [[British Army]] case-officers from the Department of Military Intelligence (DMI), for the Army's so-called "Silent Section", otherwise known as M04(x). Quickly trained by MI5 veterans at [[Cavalry Barracks, Hounslow|Hounslow Barracks]], outside London, these freshly-minted M04(x) Army case-officers were deployed to [[Dublin]] beginning in the spring of 1919.  Over time, 175 officers were trained and dispatched to Ireland. In Ireland, they came under the command of General [[Cecil Romer]] and his Deputy, Lieutenant Colonel Stephen Searle Hill-Dillon.<ref>{{Cite web|url= http://www.BloodySunday.co.uk/castle-intelligence/hill-dillon/hill-dillon.html |title= Stephen Searle Hill-Dillon|website= www.BloodySunday.co.uk|publisher= Bloody Sunday|access-date= 21 November 2018}}</ref>

In April 1919, Colonel Walter Wilson of the Department of Military Intelligence arrived in Dublin to take over the day-to-day management of these 175 Army intelligence-officers, and the unit was designated as the "Dublin District Special Branch" (DMI/MO4(x)/DDSB), because it operated exclusively within the confines of the Army's Dublin Military District. Royal Marine Colonel [[Hugh Montgomery (Royal Marines officer)|Hugh Montgomery]] of the Department of Naval Intelligence, was also seconded to Romer's intelligence staff at this time.<ref name="Hittle">{{Cite book|first= J. B. E.|last= Hittle|author-link= J.B.E. Hittle|title= Michael Collins and the Anglo-Irish War: Britain's Failed Counterinsurgency|location= Washington, D.C.|publisher= Potomac Books|year= 2011|isbn= 978-1-59797-535-3}}</ref> British Army after-action reports and contemporary accounts indicate that M04(x)/DDSB was considered by some a highly amateurish outfit. Serious cover constraints, coupled with alcohol abuse and social fraternisation with local prostitutes would prove the downfall of several of these amateur sleuths.<ref name="Hittle" /> Despite these failings, it was not MI5, but one of Basil Thomson's agents, John Charles Byrnes, a [[double agent]] within the IRA, who identified [[Michael Collins (Irish leader)|Michael Collins]], and came close to arranging his capture. The IRA identified Byrnes as a British spy and murdered him in March 1920.<ref>{{Cite web |url= http://www.BloodySunday.co.uk/castle-intelligence/thomson/byrnes/charles-byrnes.html|title=John Charles Byrnes or Jack Jameson|website=www.BloodySunday.co.uk}}</ref>

The intelligence staff of Michael Collins [[Irish Republican Army]] penetrated the unit.<ref>{{Cite book|first= T. Ryle|last=Dwyer|year=2005|title=The Squad and the Intelligence Operations of Michael Collins|location= Cork |publisher=[[Mercier Press]]|isbn=978-1-85635-469-1}}</ref>  Using DMP detectives [[Ned Broy]] and [[David Nelligan]], Michael Collins was able to learn the names and lodgings of the M04(x) agents, referred to by IRA operatives as "The Cairo Gang". On "[[Bloody Sunday (1920)|Bloody Sunday]]", 21 November 1920, Collins ordered his counter-intelligence unit, [[The Squad (IRA unit)|The Squad]], to assassinate 25 M04(x) agents, several British courts-martial officers, at least one agent reporting to Basil Thomson, and several intelligence officers attached to the [[Royal Irish Constabulary Auxiliary Division]], at their lodgings throughout Dublin.<ref name="Hittle" /> That afternoon, a mixed force of the British Army, the [[Royal Irish Constabulary]], and the [[Black and Tans]] retaliated by indiscriminately shooting dead 14 civilians at a [[Gaelic Football]] match at [[Croke Park]].<ref>{{Cite news|url= https://www.BBC.co.uk/news/world-europe-13434653|title= Croke Park: Queen in emotionally charged visit|publisher=[[BBC News]]|date=18 May 2011|access-date=1 July 2012}}</ref> 

The net impact of Collins's strike of Bloody Sunday, 21 November 1920, was relatively negligible, even though the IRA had not gone up against MI5 professionals, but instead only against a quickly trained outfit of amateur army "D-Listers".<ref name="Hittle" /> Although the shooting of 14 British officers had the desired effect on British morale, other aspects were botched by the IRA. Three of Collins's men were apprehended after engaging in a shoot-out on the street, and at least two of the wounded British officers had no connection whatsoever to British intelligence. Moreover, with MO4(x) having fielded a total of 175 agents of the DDSB, Collins's operation only temporarily slowed British momentum.  Within days, the remaining 160-odd M04(x) agents were re-established in secure quarters inside solidly loyalist hotels in Dublin, from where they continued to pursue Collins and the IRA relentlessly right up until the truce of July 1921.<ref name="Hittle" />

In December 1920, the entire DDSB was transferred from [[British Army]] command to civil command under Deputy Police Commissioner General Ormonde Winter, and thereafter was known as "D Branch" within Dublin Castle. By January 1921, the highly experienced [[MI6]] operative David Boyle arrived at Dublin Castle to take over the day-to-day management of D Branch.  The unit's former commander, Colonel Wilson, resigned in protest against having had his command taken from him. D Branch thrived under Boyle's leadership.<ref name="Hittle" /> 

In 1921, Sir [[Warren Fisher (civil servant)|Warren Fisher]], the government inspector-general for civil-service affairs, conducted a thorough review of the operations and expenditures of Basil Thomson's Home Intelligence Directorate. He issued a scathing report, accusing Thomson of wasting both money and resources, and conducting redundant as well as ineffectual operations. Shortly thereafter, in a private meeting with Prime Minister [[David Lloyd George]], Sir Basil Thomson was sacked, and the Home Intelligence Directorate was formally abolished.  With Thomson out of the way, [[Special Branch]] was returned to the command of the Commissioner of The Criminal Investigation Division at [[Scotland Yard]]. Only then was Vernon Kell able once again to rebuild MI5 and re-establish it in its former place as Britain's chief domestic spy agency.<ref name=Hittle/>

MI5 operated in Italy during inter-war period, and helped [[Benito Mussolini]] get his start in politics with a £100 weekly wage.<ref name=Guardian2009-10-13>{{Cite news|first=Tom|last=Kington|date=13 October 2009|title= Recruited by MI5: the name's Mussolini. Benito Mussolini |url=https://www.TheGuardian.com/world/2009/oct/13/benito-mussolini-recruited-mi5-italy|website=[[The Guardian]]|location=London|access-date=14 October 2009}}</ref> MI5's efficiency in counter-espionage declined from the 1930s. It was to some extent a victim of its own success as it was unable to break the ways of thinking it had evolved in the 1910s and 1920s. In particular, it was unable to adjust to the new methods of the Soviet intelligence services, the [[NKVD|People's Commissariat for Internal Affairs]] (NKVD) and [[GRU (G.U.)|Main Intelligence Directorate]] (GRU). It continued to think in terms of agents who would attempt to gather information simply through observation or bribery, or to agitate within labour organisations and the armed services, while posing as ordinary citizens. The NKVD, meanwhile, had evolved more sophisticated methods; it began to recruit agents from within the [[upper classes]] (most notably from [[University of Cambridge|Cambridge University]]), whom it regarded as a long-term investment. Such NKVD agents succeeded in gaining positions within the government, and, in [[Kim Philby |Kim Philby's]] case, within British intelligence itself, from where they were able to provide the NKVD with sensitive information.  The most successful of these agents; [[Kim Philby|Harold 'Kim' Philby]], [[Donald Duart Maclean|Donald Maclean]], [[Guy Burgess]], [[Anthony Blunt]], and [[John Cairncross]]; went undetected until after the [[World War II|Second World War]], and became known as the [[Cambridge Five]].<ref name="CambridgeBBC">{{Cite web|url= https://www.BBC.co.uk/history/historic_figures/spies_cambridge.shtml|title=Historic Figures: The Cambridge Spies |publisher=[[BBC|British Broadcasting Corporation]]|access-date=1 July 2012}}</ref>