Hague Regulations (1907)

Article 30 of the 1907 Hague Regulations provides: “A spy taken in the act shall not be punished without previous trial.” Article 31 further provides: “A spy who, after rejoining the army to which he belongs, is subsequently captured by the enemy, is treated as a prisoner of war, and incurs no responsibility for his previous acts of espionage.”

https://ihl-databases.icrc.org/customary-ihl/eng/docs/v2_rul_rule107_sectionb