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spy; secret agent
敵国の間諜が国内に潜伏していた。
Spies from the enemy nation were hiding in the country.
間諜活動が発覚し外交問題になった。
The discovery of espionage became a diplomatic issue.
間諜の正体を暴くのは困難だった。
Uncovering the spy's true identity was difficult.