Significant stay-behind operations existed during World War II, in which both the United Kingdom and Nazi Germany put in place such structures (the Auxiliary Units and the Werwolf organisation, respectively). During the Cold War, the NATO and CIA sponsored stay-behind forces 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 the Communist party came to power in a democratic election. 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 country which they deem to be subversive — rather than fighting an outright invasion, they claimed to be fighting a quieter subversion of their country. Some have been involved in domestic terrorism campaigns, such as Gladio's participation in Italy's strategy of tension and support for far-right military coups in Greece (the "Regime of the Colonels") and in Turkey. In this last country, General Kenan Evren, who staged the 1980 military coup, was a member of the "Counter-Guerrilla", Turkish stay-behind armies. 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 armée secrète included many members of its country's stay-behind organisation.