The popular thing for both pundits and politicians to do is blame the present state of affairs in Iraq on the prosecution of the occupation, rather than the acknowledge that the invasion was the mistake. The truth is that regardless of the course of action we chose after the invasion, we would have ultimately ended up with a sectarian conflict. The most popular argument is that when Paul Bremer disbanded the Iraqi Army, he set in motion the dynamics which created the Sunni insurgency. I would argue that had he not, a sectarian conflict would have began even faster than it eventually did. Saddam's Army was directed almost entirely be Sunnis who were loyal to his regime. Had we kept that group together they most surely would have inflicted the exact same terror tactics against the Shia that the Shiite run Iraqi Police Force currently inflicts on the Sunni population.
The truth is that we plunged an entire nation into Anarchy betting that the default state of political existence would automatically be democracy. How foolish we were. The default state of politics in an anarchic environment is anarchy. The destruction of the rule of law, no matter how flawed, was guaranteed to create an environment where people resorted to methods of basic survival. In those circumstances human beings are capable of horrors they would never even consider in an environment where some modicum of order existed. The instant we marched across the border this things was over, just as was predicted by many Arab leaders. Now, everyone but the Bush Administration realizes that the game is over. Those who supported this misadventure can't possibly face the reality that the problem was the invasion and not the aftermath, because they would certainly have to shoulder a lot more responsibility.
How hard was this to predict? The Russians experienced it, the Brits experienced it, and the French found out yet we still marched directly into the Gates of Hell. A fool learns from his own mistakes, a wise man learns from the mistakes of those before him.