An interesting analysis

The irrelevance of America's withdrawal from Iraq

Foreign Policy in Focus reports (April 2^nd):  Iraq today remains a violent, poorly institutionalized place with deep societal fissures and unresolved political tensions.  But little has happened in the months since the U.S. withdrawal which differs significantly from what had been happening while the U.S. remained. The negative trends are the same ones which plagued Iraq despite the presence of U.S. troops in 2007, 2008, 2009, 2010, and 2011. The U.S. presence contributed to some of those problems, helped deal with some, and failed to resolve others. 

But the key point is that extending the U.S. presence beyond 2011 would likely have had almost no impact on any of these trends.  By serving as a lightning rod for political criticism in a very hostile Iraqi political arena, an unpopular extension might well have made them worse.  The argument that the U.S. would have more influence over Iraqi politics if it had not withdrawn its troops simply has very little foundation.

http://lynch.foreignpolicy.com/posts/2012/04/02/americas_withdrawal_from_iraq_didnt_matter