Why do European Union (EU) member states sometimes respond collectively to prevent or address large-scale humanitarian crises while, at other moments, they use different institutional channels? More than once, EU states have pondered, hesitated, disagreed and let others interfere when widespread and systematic killing of civilians were looming. Instead of using the EU’s military crisis management capacities, member states have acted through different institutional channels such as the North Atlantic Treaty Organization (NATO), ad-hoc coalitions of states or single state-led operations to interfere in humanitarian crises. At times, they have decided not to intervene at all. Why does Europeans’ involvement in humanitarian intervention vary so strikingly? To examine this striking variation in European states’ responses to large-scale humanitarian crises, the thesis draws on in-depth case study evidence from the conflict in Libya during 2011, the post-electoral crisis in Côte d’Ivoire during 2010/2011, the sectarian war in the Central African Republic during 2013 and 2014 and the fight against Boko Haram in Nigeria and the Lake Chad region. The cases capture the entire range of variation on the dependent variable covering EU operations, NATO operations, ad-hoc operations, and non-intervention. The thesis develops a three-step model to explain why, when, and how European states use military force for humanitarian purposes. The model is situated at the intersection of domestic preferences and the international opportunities and constraints under which European states seek to realize their foreign policy goals. The findings show that, in combination, these factors condition European states’ readiness to intervene. Hence, a preference for non-intervention is easier to maintain if others are willing to intervene, but more difficult to pursue if the resort to force is urgent and the non-European actors are unable or unwilling to offer an appropriate response. At the regional European level, states’ power resources and preferences influence the institutional channel through which European states ultimately decide to intervene militarily. The findings show that the deployment of EU and NATO operations is likely when member states’ preferences are at least weakly congruent and backed by the interests and preferences of the organizations’ most powerful states. Diverging preferences among member states severely hinder common military operations and compel states to resort to ad-hoc arrangements. The dissertation concludes that European states’ preferences, the political contexts in which they operate and their ability to pursue their goals at the international and the regional level considerably influence why, when, and in which format European states intervene in humanitarian crises.