In a stay-behind operation, a country places secret operatives or organizations in its own territory, for use in the event that an enemy occupies that territory. If this occurs, the operatives would then form the basis of a resistance movement or act as spies from behind enemy lines. Small-scale operations may cover discrete areas, but larger stay-behind operations envisage reacting to the conquest of entire countries.
Stay-behind operations of significant size existed during World War II. The United Kingdom put in place the Auxiliary Units. Partisans in Axis-occupied Soviet territory in the early 1940s operated with a stay-behind element.
During the Cold War, the North Atlantic Treaty Organisation (NATO) and the Central Intelligence Agency (CIA) sponsored stay-behind networks in many European countries, intending to activate them in the event of that country being taken over by the Warsaw Pact or if a communist party came to power in a democratic election. According to Martin Packard they were "financed, armed, and trained in covert resistance activities, including assassination, political provocation and disinformation."
Many hidden weapons caches were found in Italy, Austria, Germany, the Netherlands and other countries, at the disposition of these "secret armies". The most famous of these NATO operations was Operation Gladio, acknowledged by Italian Prime Minister Giulio Andreotti on October 24, 1990.
The Pripet Swamp, which created a great gap between German Army Group Center and Army Group North soon after ... June 1941, made it impossible for large military formations to conduct mutually supporting operations. Attempts to bypass such extensive wetlands proved perilous, because outflanked Soviet stay-behind forces and partisans pounced on logistical troops as soon as German spearheads disappeared.
The Finns soon became seriously hindered and harassed by Soviet forces operating in their rear areas. Some of these units were left to operate as stay-behind or partisan units as the Soviets retreated.