behind

A stay-behind operation is one where a country emplaces secret operatives or organizations in its own territory, for use in case of a later enemy occupation. The stay-behind operatives would then form the basis of a resistance movement, and 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 whole countries.
Stay-behind also refers to a military tactic whereby specially trained soldiers let themselves be overrun by enemy forces in order to conduct intelligence, surveillance, target acquisition, and reconnaissance tasks often from pre-prepared hides.

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