President Bush agreed to pull U.S. combat forces out of Iraq by the end of 2011. President Obama accepted that goal. From the point of view of the U.S., the object was to leave behind a stable Iraqi government. They expected this government to include Shiites and Sunnis and Kurds.
Both countries believed that the United States would maintain a presence in Iraq by continuing to train the Iraqi army and provide U.S. air power to protect the country. These efforts were intended to bring stability to the new government.
In the end, none of these things happened.
Again and again, the Iraqi government rejected plans for U.S. support. The proposed number of U.S. soldiers went down from 16,000 to 3,500, and only a dozen fighter jets. The Iraqis accepted this. But the U.S. refused to leave any troops on the ground if they were going to be subject to prosecution by Iraq for any crimes they committed, such as causing the death of civilians.
There was no agreement. President Obama announced the mission completed and ordered all troops to return home. U.S. critics believe the Iraqi government was unwilling to take the political risk of agreeing to U.S. proposals and troops.
The price of the U.S. pulling out is being felt in different ways. Now, that no U.S. planes are present and without an Iraqi air force, Iranian planes are delivering supplies to Syria. Al Quaeda is active along the Syrian border and Turkish planes are attacking Kurdish opponents in the mountains.
Many lives and a tremendous amount of money were spent in Iraq. It does not look like the U.S. has much to show for it.