This paper develops propositions on how to theorise the mechanisms linking three particular phenomena: 1) organised civil violence, framed by perpetrators and victims as being ‘ethnic’, 2) the involvement of groups in such violence having putative ethnic kin in a neighbouring country, and 3) the choice by the executive in kin countries to escalate the neighbouring civil violence by intervening militarily in support of a conflict party. The ‘first actors’ in this analysis are the members of the executive in the intervening state. In developing a causal narrative connecting transnational ethnic ties with the choice to intervene, this paper seeks to take account of the socially constructed nature of ethnic identity, and the extent to which the politicisation of ethnic identity is endogenous to organised political violence. In order to link processes of construction with mechanisms of choice, the paper argues that social-theoretic ‘bridge building’ is a useful analytical tool – ‘bridge building’ being an approach that typically combines rationalist and conventional constructivist modes of social explanation to build more complete theories of political outcomes.