The US (read: 'Obawma administration') never should have withdrawn troops from Iraq.
I'm not saying that I was right about Iraq from the start. I fell for the fake satellite photo just like Bush II did. And I had no reason to doubt it. Bush II, a blandly anti-intellectual combination of moral crusader and benevolent dictator, supported the US (and probably British) war and oil industries. If you go looking for a war to fight, you will surely find one in the Middle East.
However, as with most nations, the situation in Iraq is and was more complex than it seemed at first glance. Sunnis and Shias are fighting an internal Jihad to determine which vaguely justifiable interpretation of the Quran shall rule Iraq. Saddam Hussein was an external threat, but the real threat was cultural, not political.
The Bush II administration knew about the cultural conflict, but chose to ignore it in favor of fighting the easier battle, the one they knew the US could easily win. Soon after I learned about the cultural conflict, I realized that eliminating Hussein will actually do the Jihadists a huge favor. The real threat to US national security is the Jihadists, not Saddam Hussein. I knew this long before the Iraq II, yet I was duped by "whoever" into believing that Hussein was stockpiling WMDs, thus making Hussein the greater threat. (I was also disgruntled with UN inspectors in Iraq whom I was led to believe were being paid to look the other way.)
Once the invasion was completed, it was the US's (i.e., the Omama administration's) duty to keep troops in Iraq until true stability and self-reliance was obtained. Instead, in 2011, Obammy chose to divest himself of all responsibility for the situation in Iraq and blow sunshine up the ass of the entire world by lying and declaring that stability and national self-reliance was achieved.
This year, the Obammy administration was fully aware that Iraqi Jihadists were encamped in Iraq while they prepared for a new assault on their cultural opposition. But the Oblammy administration chose to ignore this threat - of course. Because the official position of this Presidency is that Iraq is stable and self-reliant. A stable and self-reliant Iraq is not prone to such things, although all the evidence pointed to the fact that Iraqi police don't know which part of a gun to hold onto, and furthermore, that they have nothing left to defend, culturally speaking, "democracy" being only an empty abstraction to them relative to 1500 years of Islamic history and its powerful psychological and cultural influence.
So of course the half-hearted and morally vacuous Iraq police and army abandoned their posts. They have nothing to lose and everything to gain.