This article considers whether there is any lawful authority for foreign armed forces assisting a territorial State during a non-international armed conflict to arrest and detain civilians. Taking the backdrop of Iraq and Afghanistan it considers relevant UN Security Council resolutions including Resolution 1546 (2004) relating to Iraq which authorized the multi-national force (MNF) ‘to take all necessary measures’ and provided for the internment, for imperative reasons of security, of civilians. In respect of Afghanistan, a number of resolutions authorized the International Assistance Stabilisation Force (ISAF) to ‘take all necessary measures’. It challenges the notion that the positive rights under international humanitarian law applicable to an international armed conflict apply, mutatis mutandis, to a non-international armed conflict, where national law (including human rights law having extra-territorial effect) is of primary (although not of exclusive) significance. It also considers which body of national law, that of the sending or that of the receiving State, applies to determine the lawfulness of detention of foreign civilians. The article recognizes that the arrest and detention of civilians may be necessary during a non-international armed conflict but concludes that the lawful justification for doing so needs to be clearly established.