Testifying before the Senate Armed Services Committee, Gen. David Petraeus says the U.S. has stopped the Taliban's momentum in Afghanistan, but he warns that the gains are 'fragile and reversible' and that hard fighting still lies ahead. Key senators express confidence in the war effort.The U.S. has stopped the Taliban's momentum in Afghanistan, the general leading that war told Congress on Tuesday morning, and key senators agreed that the goal of handing off security responsibilities to Afghan forces by 2014 is achievable.