Nouri al-Maliki's tenure as Iraqi prime minister was disastrous for US policy regarding Iraq. He divided the country, amplifying the divisions between the ethnic and religious groups and moved Baghdad toward Iran, Russia and even China.
Contrary to some arguments Al-Maliki's mismanagement and abuse of power did not give rise to ISIS which was already in existence. The common narrative regarding the rise of ISIS being related to the US withdrawal in 2011 is false. ISIS was already in existence and grew out of a schism within al Qaeda. Having moved the primary realm of operations to Syria it was the corrupt and brutal regime of the US installed Nouri al-Maliki that opened the doors of support for an ISIS 'surge' in Iraq. That's when it began to really make the news.
The rise of ISIS proved a convenient excuse for US intervention, and a reason for Washington to push for the removal of al Maliki. ISIS was the excuse for the US to re-enter the Iraqi theatre but on a different terms. The Rumsfeld vision of military conquest was dead but now the US could enter on a hi-tech/special ops basis with strategically located 'advisors'. The US could maintain relationships with the Iraqi military and remain in a position to influence the theatre without having tens of thousands of troops on the ground.
Washington had been in danger of losing Iraq, a great prize in the annals of US Imperialism and the struggle goes on. While ISIS has been a source of horror to the public I do not doubt there are some policy planners, strategists and military thinkers that are (in a sense) thankful for its rise. There is a certain logic to the conspiracy theory that argues the US actually facilitated its rise in order to justify expanding operations in the theatre, to let loose the dogs of war (as it were) in order to bring down Assad and justify interventions that would harm Iranian interests. This does not mean the US directly created and/or manages ISIS. As with many false conspiracy theories, there is a grain of truth.
A great deal of confusion is also generated by the misperception that these various groups function under a monolithic and united command. That's not the reality on the ground and in many cases there are temporary tactical alliances that are formed that will often defy official strategic and ideological proclamations.
As I have argued elsewhere there's a difference between deliberate creation and what could be called strategic permissiveness. The latter is no less nefarious, though it certainly affords plausible deniability.
ISIS has also benefitted US policy in its disruption of Chinese oil operations, and has certainly fractured the power base of Baghdad's Shiite regime. Over the past several years Beijing has moved into Iraq and has signed contracts giving access to the large Halfaya and Rumaila oil fields. This has enraged Washington and also exhibits the failure of the Bush strategy. They initiated a very expensive war resulting in the death of hundreds of thousands of people, tens of thousands of US casualties and for what? On one level it is Beijing, Tehran and Moscow who have benefitted the most.
But on another level, the US Imperial footprint and even the whole of US society have been transformed. I suppose one's interpretation is dependent on one's cynicism and understanding of the larger issues vis-à-vis geopolitics, energy and the need to transform US society in order to maintain a permanent imperial posture.
Al-Abadi who replaced al-Maliki has been a disappointment. He has continued to move Baghdad closer to Iran, Russia and China. This is (in part) due to the fact that the Iraqi government seems to know that the US is a less-than-honest player and is not seeking their best interests. The US media has suppressed a lot of the history of Iraq over the past 25-40 years and yet many in Iraq know it and some will intuitively know the greatest enemy for their country has been and continues to be the United States.
Al-Abadi is already in trouble as the Shiites under Moqtada al-Sadr are beginning to protest in earnest and there are also indications of schism among the various Shiite factions. The US seems to vacillate but there have been hints suggesting Washington has all but adopted a policy that intends to (ultimately) partition both Iraq and Syria. Such Balkanisation will allow the US to form secure power-bases and footholds in the region, control oil and plot the next series of wars. These divisions will also afford new opportunities to move against Chinese, Iranian and Russian interests.