Shaping institutional overlap: NATO's responses to EU security and defence initiatives since 2014

This article analyses how and when institutional actors can shape overlap with other international organisations. Growing overlap either poses the threat of marginalisation to the incumbent organisation or offers opportunities for cooperation. Institutional actors should therefore be expected to try shape the relations with the overlapping organisation to protect their own. The article theorises that institutional actors can shape overlap if they possess sufficient institutional capacity and face a favourable opportunity structure. Whether institutional actors embrace or resist overlap, in turn, depends on their perception of the nature of the domain expansion of the other international organisation. Relying on 20 interviews with senior officials, the article probes the argument against the case of the growing overlap between the North Atlantic Treaty Organization and the European Union resulting from the latter's recent security and defence initiatives. Contrary to most expectations, it finds that the North Atlantic Treaty Organization actors played a consequential role in restructuring the relationship with the European Union.