The job is to change the Iraqi regime. Saddam's regime, refused to comply with it's cease fire agreement. Few Americans who gave this much thought, saw a good possibility that Saddam would choose to honor his commitment.
So the job was to seriously threaten Saddam, and if he didn't decide to change his mind when shown this blatant possiblity that a miltary force which could change his regime, failing this to then to use this force to "change his mind" or more probably to remove him.
As a superpower or any power, we have a responsiblity to ensure that Iraq didn't devolve into utter chaos resulting in hundreds of thousand to millions deaths, which could have resulted if we had merely removed Saddam regime and left a power vacuum- civil war & drawing in regional powers.
So after the job of removing Saddam's regime, the next aspect of the job is to establish a new Iraq government which can maintain order and a good relationship to countries in the region.