ISIS has emerged as an enemy like no other in the post-Charter era, which has galvanized a broad Coalition of States to stand up in opposition against it. The Chapter argues that the Coalition’s actions against ISIS in Syria illustrate that the jus ad bellum, and in particular the right of self-defense, has adapted to meet the challenges posed by sophisticated transnational organized armed groups. At the same time, lawyers are struggling to find clear answers to a range of practical issues that have arisen under the jus in bello as it applies to Coalition operations against ISIS in Syria. The Chapter examines the reasons why the jus ad bellum appears to have evolved in a way that the jus in bello has not, and why it is important for States to engage with the law and ensure that it remains relevant to modern challenges in order to avoid the risk that international counterterrorism operations are conducted in a legal vacuum.