Khattala

DOD has confirmed that an (as-yet-unidentified) American citizen is being held in U.S. military custody in Syria or Iraq as an enemy combatant. More specifically, the available information asserts that he was a fighter for the Islamic State who was captured in Syria by U.S.-friendly forces (or at least surrendered to those forces), and was then turned over to the U.S.

How significant is this development from a legal perspective?