This story from the NYTimes magazine is important and so well written. 

The Fighter

By C.J. Chivers:

A corollary, often unstated but understood by those doing the patrols, was that any group of Afghans willing to face the Marines head to head would gradually be thinned, while every seven months the Americans, bloodied and made jumpy by firefights and bombs, would be replaced with fresh troops. It was small-scale attrition warfare, with hearts-and-minds jive.