The Rules of War, Written Down
The Geneva Conventions are the core treaties of international humanitarian law, the body of rules that seek to limit the effects of armed conflict on people who are no longer fighting, the wounded, prisoners, and civilians.
The Conventions do not attempt to outlaw war itself. Instead they set binding limits on how war can be fought, requiring humane treatment for anyone not actively fighting and placing medical personnel, hospitals, and civilians outside the scope of legitimate military targets.
Today's four Conventions, adopted in 1949, are the most widely ratified treaties in world history, accepted by every UN member state. Violating them is not merely a diplomatic breach, it is a war crime under international and, in many countries including the United States, domestic law.