Which defense union? Determining the causal impact of policy design on public preferences for European Defense Union alternatives through a randomized conjoint experiment in 5 western-European countries

Prior to the 2022 Russian Invasion of Ukraine, the European Union was beginning to make initial steps in integrating the defense capabilities of its member states. The ambition was to incrementally achieve what French President Emmanuel Macron had labeled ‘strategic autonomy’. The Russian invasion of Ukraine exposed the lack of preparedness of western EU member states in front of a serious military threat. The war has so far emphasized the essential role still played by NATO as a guarantor of security in Europe, while undermining the credibility of incremental approaches to a Europeanization of defense achieved through the progressive expansion of pre-war initiatives. Yet, from production to capabilities to strategic thinking, the war has also identified the critical bottlenecks of European defense, and has created a new momentum for defense integration that qualitatively differs from previous initiatives in the field. However, the political feasibility of such programmes remain disputed, as any EU-wide approach to defense will entail both additional financial costs and a share of responsibilities and sovereignty on the matter. Furthermore, any such policy design is inherently multidimensional, differing over scope and level, governance, source of financing among other dimensions. To determine public support for European security cooperation, we conduct the first conjoint experiment ever fielded on public support for alternative defense union designs. We field a pre-registered, randomized conjoint experiment on a highly representative sample of the French, German, Italian, Dutch and Spanish population in November 2022. This multidimensional conjoint experiment allows us to determine the causal link between policy features of potential defense pacts, and public support or opposition to such policy. Our results show that policy packages meeting the most support require higher levels of ambition, joint EU-level governance, joint purchases of military equipment through joint procurement, and progressive taxation increases as the preferred form of financing. All in all, our results not only show that there is considerable cross-border support for defense integration in western Europe, but also that citizens in different western European countries have generally converging preferences regarding the actual design of such policy, indicating that a compromise policy is feasible and publicly supported. Furthermore, our results support ongoing research on the nature of European solidarity at times of crisis, suggesting that European citizens are willing to support the creation of joint institutions and policies to face issues of common concern, and therefore indicating that major crises open important windows of opportunity to re-shape EU-level policies and institutions.