This article examines the intervention/non-intervention nexus which has confounded scholars and diplomats ever since its inception in the eighteenth century. Seven positions on non-intervention/intervention in the long nineteenth century are presented and compared with seven quasi-similar present-day positions. Then classical international political theory in the long nineteenth century is briefly broached on the basis of the dichotomy between cosmopolitanism and communitarianism. Kant's stance on the question is examined in detail, from his own restricted non-intervention premise which permits for only one exception, to the view of Kantian scholars, several of which tend to regard Kant as more open towards exceptions to intervention, including humanitarian intervention. J.S. Mill is far more detailed on the matter. Five arguments on the part of Mill are identified in favour of non-intervention, especially if the internal war is against 'native tyrants' (where it is better to save themselves without 'foreign bayonets'); and five arguments as exceptions to intervention, including struggles against foreign rule, counter-intervention and intervention for humanitarian reasons. The article concludes by pinpointing the lasting contributions of Kant and Mill on the subject discussed that are also of relevance in today's world society.