WASHINGTON — The Obama administration's decision to withdraw the bulk of U.S. troops from Iraq over the next 19 months has sparked fears that Iraq will once again plunge into the wide-scale and debilitating violence that it endured from 2004 to 2007. Those fears are, for the most part, overblown. There are good reasons to believe that the level of stability achieved in Iraq can be maintained even without a large-scale U.S. presence.