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In a stay-behind operation, a country places secret operatives or organisations in its own territory, for use in the event that the territory is overrun by an enemy. If this occurs, the operatives would then form the basis of a resistance movement, or would act as spies from behind enemy lines. Small-scale operations covered only small areas, but larger stay-behind operations envisaged entire countries being conquered.
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 that they would be activated 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 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.
In some cases, stay-behind operations have deviated from their stated purpose, and have been active against elements in their own countries which they deem to be subversive — rather than fighting an outright invasion, they claimed to be fighting a quieter subversion of their country.
In some countries, there has been a considerable degree of overlap between official stay-behind organisations and other, non-official groups — for example, the French Organisation de l'armée secrète included many members of its country's stay-behind organisation.
List of certain known stay-behind plans 
- Auxiliary Units (United Kingdom)
- Attack on the WSA (Luxembourg)
- O&I (nl) (Netherlands) (The Dutch secret stay-behind organisation was the only non connected NATO organisation in Europe.)
- Operation Gladio (Italy, although working in many countries)
- Nihtilä-Haahti plan (Finland)
- Projekt-26 (P-26, Switzerland)
- Werwolf (Nazi Germany)
- Regional Force Surveillance Units (Australia), non-secret units of the Royal Australian Corps of Infantry with officially acknowledged 'stay-behind' duties.
- Arla gryning (sv) and Informationsbyrån ('IB') in Sweden.
See also 
- Ganser, Daniele, Nato's Secret Armies: Operation Gladio and Terrorism in Western Europe, ISBN 0-7146-5607-0.