This article presents an interdisciplinary comparison of British and German legal arguments concerning the justification of the use of force against the Islamic State in Iraq and Syria (ISIS). It is situated in the broader framework of research on strategic culture and the use of international law as a tool for justifying state behaviour. Thus, a gap in political science research is analysed: addressing legal arguments as essentially political in their usage. The present work questions whether differing strategic cultures will lead to a different use of legal arguments. International legal theory and content analysis are combined to sort arguments into the categories of instrumentalism, formalism and natural law. To do so, a data set consisting of all speeches with regard to the fight against ISIS made in both parliaments until the end of 2018 is analysed. It is shown that Germany and the UK, despite their varying strategic cultures, rely on similar legal justifications to a surprisingly large extent.