Unequal Partners? Rethinking Burden Sharing in a Future European Defence Union

Abstract

This study examines burden sharing among European Union member states within the hypothetical framework of a European Defence Union (EDU). The analysis is grounded in economic theory of alliances, which conceptualizes collective defense as a public good creating conditions for free-riding behavior. This theoretical lens emphasizes the need for equitable burden sharing to sustain the effectiveness and cohesion of the alliance. A cost-benefit methodology is applied, evaluating each member state's contribution and benefits based on geographic, demographic, economic, trade, and security threat dimensions. Covering the period from 2014 to 2023 and drawing on reliable, comparable datasets, the findings reveal a persistent asymmetry: 21 of 27 EU member states contribute less than the security and economic benefits they receive, while six—most notably France, Germany, and Italy—bear a disproportionately high share of the collective defense burden. The analysis also considers the impact of Brexit, highlighting the United Kingdom's role as a stabilizing factor in the equilibrium of defense burden distribution. Its withdrawal exacerbated existing disparities, increasing the relative burden on major contributors. The results underscore the urgent need for compensatory mechanisms, flexible contribution schemes, and differentiated responsibilities to ensure the long-term viability, fairness, and institutional legitimacy of any future European defense arrangement.