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Instructions for the Government of Armies of the United States in the Field (Lieber Code). 24 April 1863.
Section VI : Exchange of prisoners -- Flags of truce -- Flags of protection - Art. 114.
Art. 114. If it be discovered, and fairly proved, that a flag of truce has been abused for surreptitiously obtaining military knowledge, the bearer of the flag thus abusing his sacred character is deemed a spy.
So sacred is the character of a flag of truce, and so necessary is its sacredness, that while its abuse is an especially heinous offense, great caution is requisite, on the other hand, in convicting the bearer of a flag of truce as a spy.