AP Photo

“I hope people who say this war is unwinnable see stories like this. This is what winning in a counterinsurgency looks like.”

Lt. Col. William F. McCollough, commander of the First Battalion, Fifth Marine Regiment, is walking me around the center of Nawa, a poor, rural district in southern Afghanistan’s strategically vital Helmand River Valley. His Marines, who now number more than 1,000, arrived in June to clear out the Taliban stronghold. Two weeks of hard fighting killed two Marines and wounded 70 more but drove out the insurgents. Since then the colonel’s men, working with 400 Afghan soldiers and 100 policemen, have established a “security bubble” around Nawa.

Colonel McCollough recalls that when they first arrived the bazaar was...