The question when the use of force in self-defence against a non-state actor within the territory of another state is lawful, can be separated in two parts, namely the application of the self-defence framework against the non-state actor, which allows the defending state to assess whether force can be used against the non-state actor as an entity, and the ‘second layer’ of the principles of necessity and proportionality, which allows the defending state to assess whether force can be used within the territory of another state. The case-study of Al-Qaeda shows that the requirements of Article 51 can be applied to non-state actors, and in the case of Al-Qaeda the use of force in self-defence could be seen as lawful because the events of 9/11 qualified as an armed attack, the US reported the use of force to the Security Council, and the measures the Security Council had undertaken against international terrorism did not curtail the right to self-defence. This paper examines the cases of Afghanistan and Pakistan in order to extrapolate guidelines concerning the ‘second layer’ of the principles of necessity and proportionality, in particular the ‘unable or unwilling’ principle. It follows that this principle can be applied through a 5-step test, which include an assessment of the territorial state’s obligations under international law, the duty to request the territorial state to take measures to prevent the terrorist activities in its territory, to give the territorial state time to comply with these demands, and if the territorial state continues to fail, the defending state should assess the territorial state’s control and capacity in the region from which the threat is emanating as well as the relations between (elements of) the territorial state’s government and/or military and the non-state actor.