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Instructions for the Government of Armies of the United States in the Field (Lieber Code). 24 April 1863.
Section V : Safe-conduct -- Spies -- War-traitors -- Captured messengers -- Abuse of the flag of truce - Art. 98.
Art. 98. All unauthorized or secret communication with the enemy is considered treasonable by the law of war.
Foreign residents in an invaded or occupied territory, or foreign visitors in the same, can claim no immunity from this law. They may communicate with foreign parts, or with the inhabitants of the hostile country, so far as the military authority permits, but no further. Instant expulsion from the occupied territory would be the very least punishment for the infraction of this rule.