Despite the frequent failure of mediation efforts, the potential negative impact of this outcome on conflict dynamics in civil wars has not been systematically studied. Starting from the assumption that the failure of mediation may cause the conflict parties to conclude that non-violent strategies are ineffective, we develop a rationalist model to explain under what conditions mediation failure might lead to the escalation of violence. Shifts in the degree of negotiability of the conflict parties' goals and in the balance of their internal structure (relative strength of hardliners and moderates) are taken as explanatory variables. We test our hypotheses with a qualitative comparison of 23 cases of failed civil war mediation from around the globe. While shifting goals are rarely observed, it is shown how the strengthening of hardliners in the wake of mediation failure has significant explanatory power.