Gabor Rona is correct, of course, that (absent a later-in-time statutory override) U.S. use of force must comply with international law, regardless of the identity of the U.S. personnel involved. That’s why Brian Egan, on Friday, emphasized that “the United States complies with the international law of armed conflict . . . in all armed conflicts,” “across the board.”
Gabor is also correct that the Bush Administration at one time “spent considerable energy trying to convince itself and the world that the law of armed conflict prohibits civilian participation in hostilities.” That mistaken argument, however, is decidedly not the United States’s current view — to the contrary (see pp. 33-34 n.34). Al Qaeda forces who engage in hostilities against U.S. forces, for example, are not “unlawful enemy combatants” under international law, but they are “unprivileged enemy belligerents,” in the sense that they, like other civilians engaged in hostilities, do not thereby violate the LOAC, but they can be prosecuted for domestic-law offenses by virtue of their hostilities. Adherence to this distinction is not, contra Gabor, “stubborn” — it is correct and important.
Much more on this topic in this 2013 post.