The chapter discusses the legality of the use of force which occurred in the framework of the Syrian conflict. Particularly exhaustive are discussions regarding two issues: first, the right of State A to use force in self-defence against non-state actors operating in State B, without State B’s consent (such as in the context of US actions against ISIS in Syria); second, the right of humanitarian intervention, or the responsibility to protect, as it is sometimes called. The chapter critically reviews current literature on these two subjects and applies them to the Syrian context. Additionally, the chapter covers the less frequently discussed subject of the exclusive right of the recognised government to ask for international support. Specifically, it suggests that current discussions of these issues are deficient because we lack a full understanding of the role of jus ad bellum in armed conflicts that are intra-state and transnational (that is between a state and a non-state actor in another country). Recent conflicts have raised awareness of this deficiency. This chapter will delineate the emerging general principles regarding the role of jus ad bellum in intra-state conflicts in general and in Syria in particular.