NAWA, Afghanistan – Taliban militants were nowhere in sight as the columns of U.S. Marines walked a third straight day across southern Afghanistan. But the desert heat proved an enemy in its own right, with several troops falling victim Saturday to temperatures topping 100 degrees Fahrenheit. The Marines carry 50-100 pounds (23-45 kilograms) on their backs. But because they are marching through farmland on foot, they can't carry nearly as much water as their thirst demands. Few even realized the date was July 4, ... a world away from the strenuous task Bravo Company of the 1st Battalion, 5th Marine...