AL-JAZIRAH DESERT, Iraq – Two young cavalry leaders rode hard through the western border country, pushing their Fort Lewis troops nearly 185 miles from Tal Afar to the Euphrates River, then back again. Their faces caked with dust stuck to a week’s worth of sweat, Capts. Keith Walters and Vince Maykovich watched the horizon unfold at 35 mph from the rear hatches of their Stryker. Their troop of 15 vehicles rumbled over scrubland and dry lake beds, slowing now and then to clamber across the wadis – narrow desert gorges where water flows during the rainy season. The longtime friends...