We find a significant, sharp, and timely decline of insurgent violence in the initial phase: the security transfer to Afghan forces. We find that this is followed by a significant surge in violence in the second phase: 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 to capitalize on the reduced foreign military presence afterward.

Here is the full research article by Thiemo Fetzer, et.al.