Lieber Code

The Lieber Code of 24th of April, 1863, also known as General Order Number 100, was an instruction to the Union Forces of the USA during the Civil War that dictated how soldiers should conduct themselves at war time. The main sections were concerned with Martial Law, Military Jurisdiction, treatment of spies and deserters, and how prisoners of war should be treated. It was the first expressly codified law that expressly forbade giving "no quarter" to the enemy (killing prisoners of war), except in such cases when the survival of the unit that held these prisoners was threatened. As such, it is widely considered to be the precursor to the Geneva Convention.

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The full text of Lieber Code can be found at here.