Why should members of societies engaging in humanitarian intervention support the costs of that project? It is sometimes argued that only a theory of natural duty can require their support and that contractualist theories fail because they are exclusionary. This article argues that, on the contrary, natural duty is inadequate as a basis and that contractualism provides a basis for placing support for (justified) interventions among the duties of citizenship. The duty to support intervention is not, therefore, a competitor (of (...) indeterminate weight) to our duties to compatriots, since it rests on the same basis. That is because exclusive citizenship can be justified only if social contracts can be iterated elsewhere and successful societies therefore owe assistance to societies that are immobilized by state abuse or state failure. (shrink)