Strategic Insights: Would a Post-2011 Residual U.S. Force in Iraq Have Changed Anything? [open pdf - 240KB]
"Currently, U.S. policy analysts and governmental leaders are examining the rise of the Islamic State (IS) organization, particularly its seizure of vast expanses of Iraqi territory in the summer of 2014. People legitimately ask what could have been done and would a residual U.S. force in Iraq have prevented the spread of IS from Syria to Iraq or at least its seizure of northern Iraq? Opponents of the decision to withdraw all U.S. forces often contend that a U.S. residual force could have prevented or mitigated the IS offensive in northern Iraq. Supporters of the decision to withdraw usually point out that the Iraqi government would not agree to a Status of Forces agreement (SOFA) that allowed U.S. forces to remain in that country without being subordinate to Iraqi domestic law. The second argument seems to accept the views of the critics, while suggesting that the withdrawal was required as part of an effort to respect Iraqi sovereignty. Both sides seem to agree that a residual force in Iraq was a good idea. They disagree on why it did not occur."
The U.S. Army War College Strategic Studies Institute: http://www.strategicstudiesinstitute.army.mil/