Activities and operations designed to detect, prevent, and stop spying or intelligence gathering by enemy agents or foreign nations.
From counter- (meaning 'against') + espionage (from French, meaning 'spying'). The term emerged in the early 20th century during international tensions when nations needed defensive strategies against enemy intelligence operations.
During the Cold War, counterespionage operations became so sophisticated that the CIA and KGB created elaborate double-agent schemes where spies pretended to work for both sides—some agents didn't even know which country was really paying them!
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