From the Summary: "The Obama Administration began with a security environment in Iraq vastly improved over that which prevailed during 2005-2007. The 'turnaround' has been widely attributed to the 'troop surge' announced by President Bush on January 10, 2007 ('New Way Forward'). Recent Defense Department reports assess that overall violence is down about 90% from late 2007 levels, to levels not seen since 2003. On February 27, 2009, President Obama announced that all U.S. combat brigades would be withdrawn by August 31, 2010, leaving a residual presence of 35,000 -- 50,000 U.S. trainers, advisers, and mentors, although some might still be in combat in some circumstances. Some believe that a large U.S. presence still is required to ensure further political progress and produce a unified, democratic Iraq that can govern and defend itself and is an ally in the war on terror, and there is concern that violence might increase as U.S. troops are drawn down. A U.S.-Iraq status of forces agreement (SOFA), ratified by Iraq's parliament on November 27, 2008, mandates a complete U.S. withdrawal by the end of 2011, although it is possible that this time frame could be renegotiated if President Obama decides a U.S. presence is still needed to secure Iraq."
CRS Report for Congress, RL31339