This chapter explicates the various ways in which contemporary warfare challenges post-1945 international law on the use of force and the conduct of war. It begins by exploring the rules governing the use of force against non-state actors. This is one of the most pressing issues of the war on terror, much of which has involved military operations against terrorist groups operating from the territory of states that cannot or will not suppress their activities. In particular, campaigns by the US and several other states against ISIS in Syria have seriously undermined the international law framework governing self-defence and the right of states to have their sovereignty and territorial integrity respected. The chapter then looks at another trademark policy of the war on terror: the use of targeted killings, often carried out by unmanned drones, to eliminate suspected terrorists. It also considers a new type of warfare altogether: the emerging phenomenon of cyber warfare, which, too, has implications for both jus ad bellum and jus in bello.