This article assesses the implications of the current Syria situation for the international law on the use of defensive force against non-State actors. The law in this area is highly unsettled, with multiple legal positions in play. After mapping the legal terrain, the article shows that the Syria situation accentuates three preexisting trends. First, the claim that international law absolutely prohibits the use of defensive force against non-State actors is increasingly difficult to sustain. States, on the whole, have supported the operation against the so-called Islamic State in Syria. Second, States still have not coalesced around a legal standard on when such force is lawful. Most States seem conflicted or uncertain on that question. Third, this ambivalence has contributed to a sizable gap between the norms that are most often articulated as law and the ones that are operational. States regularly tolerate operations that they are unwilling to legitimize with legal language.
Hakimi, Monica. "Defensive Force against Non-State Actors: The State of Play." Int'l L. Stud. 91 (2015): 1-31.