Terrorist attacks pose a huge threat these days. The attack in Paris on 13 November 2015 has shown that. The terrorist organisation Islamic State (hereinafter: IS) claimed responsibility for the attack. France (and the USA) reacted by launching massive air strikes against the IS stronghold in Raqqa/Syria. Furthermore, France declared that it was exercising its right to self-defence, Art. 51 of the United Nations Charter (hereinafter: UNC). According to the traditional view in international law, the right to self-defence requires an armed attack by a state or alternatively, if the attack was launched by a non-state actor, that the attack is attributable to a state. The thesis illustrates that the IS is not a state under international law as it does not fulfil all four criteria of statehood. Since the 9/11-attacks in 2001, it is highly debated, whether this traditional view is still appropriate. Some commentators are of the opinion that an armed attack under Art. 51 of the UNC can also originate from a non-state actor. The thesis highlights that this view is not correct. In order to qualify the attack of a non-state actor as an armed attack under Art. 51 of the UNC it must still be attributable to a state. The thesis shows that the attack in Paris is not attributable to the Syria. Neither under the Draft Articles on the Responsibility of States for Internationally Wrongful Acts, nor under the Safe Haven-Doctrine or the Unwilling or Unable-Doctrine, whereby the focus of the thesis lies on the last mentioned doctrine. The content of the doctrine is analysed as well as the question whether or not the doctrine is already a norm of customary international law, which is not the case. The thesis then discusses a new interpretation of the rules of attribution in form of the clarified Unwilling or Unable-Doctrine. The underlying idea of this doctrine is that all states have certain counter-terrorism obligations from which due diligence obligations vis-à-vis other states arise. Based on this idea a definition of the terms 'unable' and 'unwilling' is elaborated. Although the elaborated definitions give the clarified Unwilling or Unable-Doctrine a clearer content, there is still a tension between the attacked state's right to self-defence and the host state's territorial sovereignty. In order to mitigate this tension four preconditions, based on the existing law, are developed. To render the defensive use of force by the attacked state lawful, these preconditions have to be fulfilled. The thesis concludes that the Paris attack is not attributable to Syria. Thus, there was no armed attack by Syria on France. The thesis further concludes that the current rules of attribution do not meet today's challenges, namely the threat that terrorist attacks pose and that there is an urgent need for a clarification of the rules of attribution
