When driving him from Kuwait, George H. W. Bush could have removed Saddam Hussein from power but did not, because the centerfugal ethnic and sectarian forces in Iraq that Saddam held together through fear would likely tear the country apart once he fell.
President George W. Bush decided to remove Saddam from power in the hopes of creating a pro-western nation in the heart of the Middle East, and instead saw the centerfugal ethnic and sectarian forces in Iraq that Saddam had held together through fear nearly tear the country apart.
George W. Bush had to settle for making the U. S. military responsible for keeping the country together, still ruling by force, but perhaps a more tolerable force than the previous dictator. Without a long term U.S. military presence, the country would surely collapse once again into ethnic-sectarian violence, and create a power vacuum which the terrorist theocracy of Iran would likely fill.
This disappointing plan B was more or less working by 2008, but Barack Obama took the first exit he could out of Iraq, which collapsed into more sectarian violence and fell under Iranian hegemony.
Iraq is now a failed state, run by Iranian-funded militias one the one hand, and the nihilistic doomsday cult of ISIS on the other.
This result was entirely predictable; the result not only of an imprudent application of force, but also of an imprudent refusal to apply it.
The question facing the candidates of 2016 is how they are going to bring U.S foreign policy back to the honorable realism of men like H. W. Bush, and away from the ideological fantasies of his successors.