Most people now recognize the US invasion of Iraq in 2003 to be a mistake, given that it had negative consequences in a variety of different areas (we’ll talk about that later). The invasion led to Iraq becoming a magnet for Islamic fighters from all over, unleashed the sectarian divisions that had been brutally suppressed by Saddam Hussein, led to horrific casualties among the population in the fighting that followed, and scared the neighboring countries (other than Iran), since it led to a Sunni ruled state becoming a Shia ruled state. It also put the United States in a position which was deemed comparable to Vietnam in the sense that mere military might did not lead to a winning position, and dealt huge blows to the armed forces of the US (to both the regular army and to the National Guard).
The issue about getting the army back from Iraq played a major role in the last Presidential campaign, and there is a lot of pressure on Obama to bring back the military; with majority public support being to bring back the army. For many years, it was unclear as to what the timeline for this would be ! However, in the last year or so, the US finally managed to get the right alliances (including with the Sunni minority, elements of which would have been fighting the American forces just a few months back). It was only this reduction of violence, along with fledging steps taken by the Iraqi army and police force (another self-created problem – the initial US head of the Provisional Authority had dissolved the Iraqi army as a contaminated Baathist organization without making any contingency plans). The situation in Iraq now, although not as secure as the US would have liked to see in an ideal world, is enough that the US is able to do the major symbolic action of withdrawing its forces from 6 major Iraqi cities (link to article):