'Speaking with one voice': Explaining coherence in EU external energy security policy

2017 saw an unusually high degree of coordination between the European Union (EU) and its Member States on external energy security policy. This is puzzling not only because it defies the expectations of critics who have long highlighted the failure of Member States to adopt coherent policies on the question of external energy security, but because individual Member States have strong incentives to adopt individualistic policies that undermine collective action in this policy area. This article seeks to answer two questions: how can coherence in EU external energy security policy be explained, and what does it suggest about the conditions under which the EU will be able to 'speak with one voice' in other contested external policy areas? In order to do so, I mine the existing literature for plausible explanations and identify and evaluate three separate narratives that rely on different causal mechanisms to explain greater coherence as a consequence of: (1) the gradual centralization of EU energy policy competence; (2) exogenous shocks to EU energy supply and; (3) institutional innovation under the Juncker Commission. I argue that none are independently sufficient to explain the phenomenon observed and present a novel 'synergetic account' that argues it was the unique interaction of each of the aforementioned factors that provided the conditions necessary for coherence, before concluding with a discussion of the implications of my findings for theory and policy.