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. [link 1, link 2]
AJacksonian unearthed this gem from the 1863 U.S. Army Field Manual, which was the law from 1863 to 1898. Perhaps with the return of highway robbers and pirates to the international battlefield, this language could be resurrected.
As the traditional punishment for Piracy is Life Imprisonment, and was the Noose before that, those who fit this description could be summarily subjected to those punishments.