This paper explores the presence of late medieval ius commune in Grotius’s thought on the use of force in internal strife and war, based on De iure belli ac pacis (1625). To this end, it examines Grotius’s use of ius commune sources, and considers some similar sources, which he does not actually cite, but which relate to his discussion. By clarifying Grotius’s selection and use of ius commune sources, the paper intends to contribute to the achievement of a double aim: firstly, to determine the place of rebellion and civil war in De iure belli ac pacis, especially in relation to (external) war; and, secondly, to assess Grotius’s approach to the two former issues, particularly with regard to the criteria by which a distinction between rebellion and civil war can be drawn, and to the effects of this distinction.