This article contributes to a burgeoning literature on parliamentary war powers by investigating the case of the US Congress drawing on both International Relations (IR) research and traditional war powers studies. Applying a fuzzy-set qualitative comparative analysis and case study method, we examine the conditions shaping congressional assertiveness. The article shows that the lack of national security interests and divided government are important conditions for members of Congress to criticize presidential intervention policies. While previous US war powers studies focused on the influence of partisanship, this article holds that domestic as well as international factors influence congressional behavior. A short comparative case study of two US military interventions (Libya 2011, ISIS 2014–15) during the Obama presidency serves to illustrate the findings.

Those countries, including the United Kingdom, using force in Libya in 2011 have taken much greater care to ensure that their actions are underpinned by legality. This suggests a return to respect for the jus ad bellum, but as the operation against Libya unfolded it became clearer that some of the problems that undermined the legality and legitimacy of the invasion of Iraq 8 years earlier had not been avoided, which raises the question of how such operations can be kept within the strict bounds of the law.