Skimming low over the desert in helicopters with guns at the ready, American troops advanced into the Khakeran Valley in southern Afghanistan on Sunday, three months after Taliban rebels attacked police and forced them to flee. The move is the latest part of a strategy to reassert U.S.-led coalition and Afghan control over an insurgent stronghold, after a spate of attacks raised fears of an Iraq-style insurgency here. Blistering U.S. assaults against nearby mountainous rebel camps last week left 178 suspected militants dead. Up to 300 insurgents are suspected to be in the Khakeran Valley, about 220 kilometers (130...