You needed to read further. There is a section that explains that "combat" troops would leave by 31 December 2011, which Obama pulled out early, 16 December 2011. There was also a part of the agreement that allowed the U.S., with Iraqi agreement to change the mission, or re-mission the American troops from combat to training and support to the Iraqi military forces and would allow for several thousand U.S. military troops to remain in Iraq to assist their security forces to continue stabilizing the country, which had stabilized quite a bit by late 2011. The Bush administration asked Obama to continue negotiation with Iraq to make this happen to ensure a stable country after December 2011, if needed. As we have seen, it was needed and Obama was informed that we needed to remain to ensure stability by many sources, both inside the military and the government.