This chapter studies the idea that Americans and Europeans hold different views on global security. Generally, it is considered that the EU side demonstrates a stronger adherence to positivism in international law, while the US adopts policy-oriented methodologies. This translates into different views of the actual content of the UN Charter rules on the use of force, for example of the permissibility of self-defense against attacks by non-State actors, calls for new exceptions to the prohibition of the use of force and in relation to controversial issues, such as the legality of the anticipatory self-defense. The chapter offers a critical look at the alleged difference by studying the US-led military coalition fighting against the so-called Islamic State (IS) in Iraq and Syria. This reveals a gradual acceptance of the more expansionist interpretation put forward by the US. Indeed, the case of the US-led military coalition against IS is one where European States - and several other western States - originally steered clear from murky legal grounds only to find themselves ultimately embracing an extensive reading of the right of self-defense, and relying for the first time, whether explicitly or implicitly, on the controversial “unable and unwilling” doctrine to justify military operations abroad.