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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. 86.
Art. 86. All intercourse between the territories occupied by belligerent armies, whether by traffic, by letter, by travel, or in any other way, ceases. This is the general rule, to be observed without special proclamation.
Exceptions to this rule, whether by safe-conduct, or permission to trade on a small or large scale, or by exchanging malls, or by travel from one territory into the other, can take place only according to agreement approved by the government, or by the highest military authority.
Contraventions of this rule are highly punishable.