KABUL, Afghanistan — Behind an electrified fence, blast-resistant sandbags and 53 National Police outposts, the Afghan surge is well under way.

But the foot soldiers in a bowl-shaped valley about 20 miles southeast of Kabul are not fighting the Taliban, or even carrying guns. They are preparing to extract copper from one of the richest untapped deposits on earth. And they are Chinese, undertaking by far the largest foreign investment project in war-torn Afghanistan...