Recent political developments on the global scene have shed new light on established rules concerning the employment of military force while giving rise, among other things, to a reappraisal of the scope and limits of the right of self-defence. The terrorist attacks of September 2001 raised the question of whether actions by non-state actors can fall within the concept of ‘armed attack’. Those attacks were defined by UN Security Council Resolution 1368, under Article 39 of Chapter VII of the UN Charter, as ‘a threat to international peace and security’, but the ambiguous formulation left sufficient scope for upholding the prevailing view that Article 51 may only be invoked in the case of conflict between states. According to this view, which meanwhile has been contested, any resort to self-defence for legally justifying unilateral military action against terrorist organizations operating in other countries needs to be supported by evidence or argumentation that attacks perpetrated by those organizations can be attributed to a state. In defending the military campaign conducted to oust the Taliban regime in Afghanistan, the US government could credibly argue that this regime, exercising effective control over the country, was to be held accountable since it was harbouring members of al Qaeda on its territory and was actively supporting them.