The events of 11 September 2001 profoundly challenged the existing principles of international law, both as to the right of a state to use military force and as to the principles of international humanitarian law. This article assesses whether international law can provide a legal framework by which to judge state responses to acts of terrorism, the organisation and preparation of which occur in another state but where the acts are committed by non-state actors. It explores the difficulties of applying international humanitarian law in selecting military objectives when directing attacks against ‘terrorists’ and in classifying captured fighters. Finally, it considers the impact of the detention of individuals suspected of ‘terrorist’ activities on international human rights standards.