Why did the U.S. decide to keep 10,000 American troops in Afghanistan?

Why did the U.S. decide to keep 10,000 American troops in Afghanistan, and what does that mean to the current Afghan government and to the men and women who are serving there?

Sebastian Junger has directed a trilogy of films about war that includes Restrepo and Korengal . His latest, The Last Patrol, examines the difficulty in returning from combat and reentering civilian life. His film about the late photographer Tim Hetherington, Which Way is the Front Line From Here?, screened at Mountainfilm 2013, and his latest book is simply called War.