On a recent tour of the piece of southern Afghanistan that his squadron will inherit when it deploys this summer, Lt. Col. Eric Smith surveyed the remnants of past efforts against Taliban insurgents, from abandoned outposts to faded roads and former phase lines. While there, Smith’s armored vehicle struck a roadside bomb. The close call underlines the continued stakes in Afghanistan as Smith’s squadron and its parent unit, the 2nd Cavalry Regiment, prepare to deploy to Kandahar province.