To highlight the hubris of thinking that Iraq would become a Democracy within a few years we need look no further than how the Northern United States brought Democracy to the Southern United States by way of the Civil War. It's taken more than 150 years and counting to establish the laws and cultural acceptance necessary for a truly democratic society to exist, and this in a country that was connected to ours, spoke our language, and shared a core identity.
To this day a radical faction still exists in the South – fueled by racial, religious, and cultural senses of superiority – that truly believes in re-establishing the pre Civil War regime in the South. This radicalism is supported by a general sense of less radical forms of superiority that are deeply embedded in the culture, a sort of gradient that connects the radical to the status quo.
To think that the USA could unite a country as deeply divided as Iraq by eliminating the leader of its dominant faction was lunacy. Only occupation, restoration, education, and acculturation over at least 200 years could have stood a chance of working. But then, only a chance. The cultural memories of the United States are shallow, the cultural weight of the Middle east is the oldest in human history.