How do foreign powers disengage from a conflict? We study the recent largescale security transition from international troops to local forces in the context of the ongoing civil conflict in Afghanistan. We construct a new dataset that combines information on this transition process with declassified conflict outcomes and previously unreleased quarterly survey data. Our empirical design leverages the staggered roll-out of the transition onset, together with a novel instrumental variables approach to estimate the impact of the two-phase security transition. We find that the initial security transfer to Afghan forces is marked by a significant, sharp and timely decline in insurgent violence. This effect reverses with the actual physical withdrawal of foreign troops. We argue that this pattern is consistent with a signaling model, in which the insurgents reduce violence strategically to facilitate the foreign military withdrawal. Our findings clarify the destabilizing consequences of withdrawal in one of the costliest conflicts in modern history and yield potentially actionable insights for designing future security transitions.