Until recently, everyone understood that prisoners of war were treated well — as actual prisoners of war, rather than spies to be hanged — only if they followed the rules of war. From The Lieber Code Of 1863:
SECTION IV.—Partisans—Armed enemies not belonging to the hostile army—Scouts—Armed prowlers—War-rebels.81. Partisans are soldiers armed and wearing the uniform of their army, but belonging to a corps which acts detached from the main body for the purpose of making inroads into the territory occupied by the enemy. If captured they are entitled to all the privileges of the prisoner of war.
82. Men, or squads of men, who commit hostilities, whether by fighting, or inroads for destruction or plunder, or by raids of any kind, without commission, without being part and portion of the organized hostile army, and without sharing continuously in the war, but who do so with intermitting returns to their homes and avocations, or with the occasional assumption of the semblance of peaceful pursuits, divesting themselves of the character or appearance of soldiers—such men, or squads of men, are not public enemies, and therefore, if captured, are not entitled to the privileges of prisoners of war, but shall be treated summarily as highway robbers or pirates.
83. Scouts or single soldiers, if disguised in the dress of the country, or in the uniform of the army hostile to their own, employed in obtaining information, if found within or lurking about the lines of the captor, are treated as spies, and suffer death.
84. Armed prowlers, by whatever names they may be called, or persons of the enemy’s territory, who steal within the lines of the hostile army for the purpose of robbing, killing, or of destroying bridges, roads, or canals, or of robbing or destroying the mail, or of cutting the telegraph wires, are not entitled to the privileges of the prisoner of war.
85. War-rebels are persons within an occupied territory who rise in arms against the occupying or conquering army, or against the authorities established by the same. If captured, they may suffer death, whether they rise singly, in small or large bands, and whether called upon to do so by their own, but expelled, government or not. They are not prisoners of war; nor are they if discovered and secured before their conspiracy has matured to an actual rising or to armed violence.