Looks like Trump may have found a new reason to keep the US war in Afghanistan going: https://www.washingtonpost.com/news/worldviews/wp/2017/07/27/trumps-crude-view-of-afghanistan-wont-solve-u-s-s-longest-running-war/
Whether the American troops stay or go, the Afghans will still fight each others, as they have always done. Until a foreign enemy appears on the scene, then they will unite against the common enemy and send them packing.
At least that is how it has been since Alexander the Great, but now it seams like the pattern is broken.