Special Operations Executive (SOE) was a British organisation formed in 1940 to conduct espionage, sabotage and reconnaissance in German-occupied Europe and to aid local resistance movements during World War II. SOE personnel operated in all territories occupied or attacked by the Axis powers, except where demarcation lines were agreed upon with Britain's principal allies, the United States, and the Soviet Union. SOE made use of neutral territory on occasion or made plans and preparations in case neutral countries were attacked by the Axis. The organisation directly employed or controlled more than 13,000 people, of whom 3,200 were women. Both men and women served as agents in Axis-occupied countries.

The organisation was dissolved in 1946. A memorial to those who served in SOE was unveiled in 1996 on the wall of the west cloister of Westminster Abbey by the Queen Mother, and in 2009 on the Albert Embankment in London, depicting Violette Szabo. The Valençay SOE Memorial honours 91 male and 13 female SOE agents who lost their lives while working in France. The Tempsford Memorial was unveiled in 2013 by the then-Prince of Wales in Church End, Tempsford, Bedfordshire, close to the site of the former RAF Tempsford.

SOE was formed from the amalgamation of three existing secret departments, which had been formed shortly before the outbreak of the Second World War. Immediately after Germany annexed Austria (the Anschluss) in March 1938, the Foreign Office reestablished a propaganda organisation from the First World War known as the Department of Propaganda in Enemy Countries. In that earlier war, it had been known more commonly as "Crewe House," eponymously named after the building in which it operated (the present day location of the Embassy of Saudi Arabia in London), and was directed by the Viscount Northcliffe, directing as section leaders such figures as H. G. Wells and Hamilton Fyfe.

The man that Neville Chamberlain had approached to reestablish the department was the Canadian newspaper magnate Sir Campbell Stuart, formerly a member of Crewe House. However, because Stuart was the chairman of the Eastern Telegraph Company (today known as Cable & Wireless), he operated the department's headquarters out of his company's buildings at the Electra House (two buildings in London connected by underground telegraphy cables). Therefore, the new propaganda unit was known as Electra House. While they were primarily focused on the production of white propaganda, Stuart also employed the staff of both Electra House buildings to monitor the telegraphy cables belonging to all foreign embassy missions in London, because the house on the Victoria Embankment sat directly on top of their cables. Where the earlier Crewe House had been primarily focused on leaflet campaigns and the production of printed materials, Electra House was involved in the full spectrum of media production: film, radio, newspapers, and gossip. However, because Stuart held a firm refusal to have his department engaged in the production of black propaganda, other departments of the War Office, the Ministry of Information, and the Foreign Office were created to undertake these productions. Later that month, the Secret Intelligence Service (SIS, also known as MI6) formed a section known as Section D (the "D" apparently standing for "Destruction") under Major Laurence Grand, to investigate the use of sabotage, propaganda, and other irregular means to weaken an enemy. In the autumn of the same year, the War Office expanded an existing research department known as General Staff (Research) [GS (R)], and appointed Major J.C.F. Holland as its head to conduct research into guerrilla warfare. GS (R) was renamed Military Intelligence (Research) [MI(R)], in early 1939. Contrary to popular misconception, GS(R) had been created in 1936, not 1938. These three departments worked with few resources until the outbreak of war. There was much overlap between their activities. Sections D and EH duplicated much of each other's work. On the other hand, the heads of Section D and MI(R) knew each other and shared information. They agreed to a rough division of their activities; MI(R) researched irregular operations that could be undertaken by regular uniformed troops, while Section D dealt with truly undercover work.

Separated by only nine months in age, Majors Jo Holland and Laurance Grand had been childhood friends at the Rugby School, and later attended classes together at Royal Military Academy, Woolwich. Holland's secretary at the time, Joan Bright Astley, remarked later in her memoirs that the two worked together exceptionally well while developing the foundations of irregular warfare that would become SOE. Astley, herself a co-founder of SOE, would later be revealed to be one of the primary inspirations for the fictional character Miss Moneypenny in the James Bond series.

Holland's interests in guerrilla warfare began after working on an operation with T.E. Lawrence during World War I, roughly a year and a half before being shot in the chest in a pub by the Irish Republican Army (IRA) during the Irish War of Independence. It was the IRA in which Holland revered technically, going so far as to state that they had made guerrilla warfare a science in the modern age. He wrote much about the IRA at this time, and encouraged British Military Intelligence leadership to create an organization that had the effectiveness of this group. As a side project, he also created the British Commandos. Grand, meanwhile, had been involved in similar guerrilla operations serving in the Far East and India. During an operation on the Indian Frontier, Grand had devised an unorthodox tactic after discovering that Pathan tribesmen were stealing British ammunition: he ordered every tenth round to be filled with high explosive instead of cordite. Though some officers regarded the idea of this as "ungentlemanly," Admiral Hugh Sinclair had considered it precisely the kind of ingenuity and ruthlessness Britain required against future enemies. It is only in Colin Gubbins' official accounting of the story (he wrote the first official history of the SOE) that any animosity existed between the two men, which modern historians have noted to be a revisionist retelling, motivated by Gubbins' dislike of Laurance Grand. Gubbins did not share the same dislike of Holland and thought of him more as a father-figure, having been recruited and placed into command by Holland himself. Gubbins' dislike of Grand began while working at MI(R) around this time, because he did not appreciate that MI(R) was limited specifically to the domain of guerrilla warfare, while Section D was authorized to conduct guerrilla activities outside of a strictly defined warfighting capacity. After the war, Gubbins was friends with the historians M. R. D. Foot and Nigel West, and used his influence over them to sabotage the legacy of Grand's primary contributions to the war effort, downplaying Section D as merely a "think tank," where in reality, Section D had operatives stationed all over Europe, and had formed relationships with many underground resistance movements. Those operatives were then absorbed into SOE.

During the early months of the war, Section D was based first at St Ermin's Hotel in Westminster and then the Metropole Hotel near Trafalgar Square. The Section attempted unsuccessfully to sabotage deliveries of vital strategic materials to Germany from neutral countries by mining the Iron Gate on the River Danube. By the time Section D was absorbed into the newly created SOE, it employed around 300 paid officers, though its real reach was far greater, in the contract agents that it had developed throughout Europe. Section D was more successful at establishing smuggling routes from Shetland through Norway that were the foundations of what would become the Shetland bus. It also established bureaus in Yugoslavia, Scandinavia, Romania, Greece, and several other locations. Laurence Grand and Section D were also responsible for establishing the D School training house at Brickendonbury, where the first batch of newly trained SOE officers were produced by Kim Philby and Guy Burgess.

However, despite all of Section D's work on the continent, the Germans kept advancing. Some contemporaries laid this failure of the British war machine squarely at the feet of British intelligence, and especially C, however, historians after the war such as Karl-Heinz Frieser and John Keegan note that the German advance might have been impossible to halt, no matter how many intelligence operatives and paramilitary operators were deployed. As a result of the advance, and his operators and foreign contacts being increasingly cut-off from allied supply lines – Laurence Grand established supply dumps set up by Section D, arranged in regular intervals between Paris and the German border. He also organized what was called the Home Defence Scheme, a form of Stay-behind network within the UK itself in the event the country was overrun and invaded.