This article analyzes how the internationalization of civil wars influences conflict parties' consent to UN mediation processes. Illustrated by the UN mediation in Syria, I argue that internationalization influences consent directly by obstructing the advent of a costly stalemate and the parties' perception of mediation as a 'way out,' and indirectly by reducing mediators' leverage and perceived impartiality thereby limiting their tools to foster consent. The article makes three contributions. First, it presents a novel conceptual framework to understand the impact of internationalization on conflict parties' consent. Second, it provides a long-term analysis of UN mediation in Syria from 2012–2020. Third, it contributes to a broader discussion about how civil wars end. This is of particular relevance as the prioritization of a political over a military end to civil wars, which was dominant in the early post-Cold War period, is no longer unquestioned.