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Espionage, as well as other intelligence assessment, has existed since ancient times.
Contents
Pre-modern espionage
Early strategists such as Sun Tzu 孫子 stressed the need for military intelligence.
Modern times
With the Modern Age came the concept of professional police organizations, the police state and geopolitics. New intelligence methods arrived, such as imagery intelligence, signals intelligence, cryptanalysis and spy satellites.
References
History of espionage Wikipedia(Text) CC BY-SA