Abstract The evolution of ISIL and the foreign terrorist fighter phenomenon has raised many legal and practical challenges. Some of the most pressing and debated relate to the thousands of men, women and children, previously affiliated (to varying degrees) to ISIL and now held in camps in northern Syria. The call for repatriation of these persons has met mixed responses. Much of the debate has focused on policy considerations rather than examining, in depth, what international law prescribes here: that is, to what extent does international law oblige the States of origin to repatriate their nationals? This article will answer this question, looking at relevant rules of human rights law (including the emerging ‘functional approach’ to extra-territorial jurisdiction) as well as international humanitarian law. It will also examine an important principle of domestic law which is likely to play a major role in how these issues are adjudicated: namely, judicial deference to the executive in matters of foreign policy. The article argues that the international legal rules invoked in favour of an obligation to repatriate are far less prescriptive than generally asserted, while the principle of judicial deference to the executive in matters of foreign policy—particularly in cases involving national security considerations—means that national courts are likely to give States yet further latitude in this regard. For both legal and practical reasons, actions aimed at convincing national courts that a State is obliged to repatriate its nationals will not suffice to resolve these complex challenges.