Operation Gladio was the codename for clandestine "stay-behind" operations of armed resistance that were organized by the Western Union (WU) (founded in 1948), and subsequently by NATO (formed in 1949) and by the CIA (established in 1947), in collaboration with several European intelligence agencies during the Cold War. Although "Gladio" specifically refers to the Italian branch of the NATO stay-behind organizations, "Operation Gladio" is used as an informal name for all of them. Stay-behind operations were prepared in many NATO member countries, and in some neutral countries.
According to several Western European researchers, the operation involved the use of assassination, psychological warfare and false flag operations to delegitimize left-wing parties in Western European countries, and even went so far as to support anti-communist militias as they tortured communists and assassinated (in 1969) Eduardo Mondlane. However, the United States Department of State maintains that the operation served only to resist a potential invasion of Western European countries by the Soviet Union.
Following the fall of France in 1940, Winston Churchill created the Special Operations Executive (SOE) to both assist resistance movements and itself carry out sabotage and subversive operations in occupied Europe. It was revealed half a century later that SOE was complemented by a stay-behind organisation in Britain, created in extreme secrecy, to prepare for a possible invasion by Nazi Germany.
A network of resistance fighters was formed across Britain and arms caches were established. The network was recruited, in part, from the 5th (Ski) Battalion of the Scots Guards (which had originally been formed, but was not deployed, to fight alongside Finnish forces fighting the Soviet invasion of Finland). The network, which became known as the Auxiliary Units, was headed by Major Colin Gubbins – an expert in guerrilla warfare (who would later lead SOE). The units were trained, in part, by "Mad Mike" Calvert, a Royal Engineers officer who specialised in demolition by explosives and covert raiding operations.