European security and defense cooperation as well as integration has been a salient issue in the past years, illustrated, for example, by the EU’s Permanent Structured Cooperation (PESCO), which enables those 25 EU governments that wanted to participate to fund, develop and deploy armed forces together. Key political figures such as French President Emmanuel Macron have also hinted at their keen interest in advancing European defense and security cooperation, for example, by pushing the EU and Europe to become strategically autonomous from the US. Yet, little is yet known about conditions under which the lay public supports such cooperation. We examine this question in the context of a plausible scenario of EU military intervention to a country whose condition can have significant repercussions on European countries. Such an intervention can take on different forms, and we expect that public support for such a mission depends on exactly how the intervention takes place. Several different aspects of the intervention likely affect people’s preferences and we are particularly interested in five theoretically important dimensions: 1) who is in charge of approving the mission? (Trigger); 2) who intervenes? (Composition); 3) who pays the mission? (Financing); 4) who participates in the mission? (Participation); and 5) how the EU should decide the intervention? (Decision making). By randomly varying the five dimensions, we experimentally examine which aspect is most relevant to citizens and how much, and how this varies across society. The experiment will be conducted in 24 EU member states and the UK, with sample sizes of 1,500-2,000 per country.