By CNN's Nick Paton Walsh and Tim Lister

The mountains of Kunar province are as beautiful as they are deadly. High alpine valleys, sweeping views in every direction. And al Qaeda units hiding beyond the ridges that U.S. military power is unable to control.

On patrol with U.S. forces in this remote eastern corner of Afghanistan, you get a sense of the enormous challenge of pacifying this wild, uncharted territory. The only way to take the high ground is by air – a perilous operation at 9,000 feet.

The ground is vitally important though. Kunar and neighboring Nurestan province are places so rugged and untameable that NATO has entirely pulled out of the latter, and has withdrawn from the Pech valley – a key patch of terrain where Americans fought for years, unsure whether they were just facing locals angry at being invaded, or hardcore militants.

Yet this partial withdrawal – criticized by some as relinquishing ground that America must hold to secure Afghanistan's porous but vital eastern border – has enabled some of the main militants that U.S. forces came to Afghanistan to defeat, to re-emerge. FULL POST