AP Photo

The only way to reach Viper Company of the 26th Regiment, First Infantry Division, is by helicopter. When I fly in, Capt. Jimmy Howell greets me. "I'm holding a shura [meeting of village elders]," he says. "We won't be shot at until they leave." The steep-sided Korengal Valley, 70 miles northeast of Kabul, is the scene of the war's fiercest fighting, claiming 57 American lives over the past three years.

Sure enough, an hour after the elders leave the shura, 30-millimeter shells strike the outpost. Cpl. Marc Madding, an Afghan army adviser, begins firing .50 caliber rounds at the enemy position, laughing as an Afghan soldier scurries from the latrine with shells bursting behind him. Capt. Howell adjusts mortar and artillery shells on the hillside, followed by an A-10...