The European Union’s
(EU’s) increasing transformation from a civilian actor to a geopolitical and
security actor has faced numerous challenges. Some would be recognizable to any
security actor, such as staffing, equipment, or organization; but the unique
structure of the EU also creates its own idiosyncratic difficulties. In
overcoming these, the EU has demonstrated evidence of institutional learning as
twenty years of experience have driven the institutionalization of ‘best
practices’. We seek to contribute to this literature on learning in the EU’s
Common Security and Defence Policy (CSDP) by assessing the relative importance
of three features in supporting or constraining institutional experiential
learning in CSDP: institutional design, political factors, and social
practices. Relying on documentary evidence and twelve elite interviews, we
conduct a comparative analysis of CSDP missions and operations in Bosnia and
Herzegovina (BiH), Mali, and Ukraine. In doing so, we outline the key features
of the missions, their challenges, and the role that the three features played
in supporting or constraining learning. Overall, we find that political and
practice factors are the most relevant to driving learning in CSDP, while
institutional design played a key role in obstructing learning.