Due to wrong decisions at every step of the way, and due to outsourcing security to paramilitaries trained and armed by us, we have created a situation which we have no ability or resources to control. So what can we do in Iraq? John Robb at Global Guerrillas sums it up:

What can we do? Nothing but leave. We can neither expect the leadership of US military to develop sound strategies for mitigating the damage done, nor can we reverse drivers of chaos that have been initiated over the last three years. This chaotic system is now running smoothly under the power of its own internal dynamics and continued intervention will only continue to worsen it. Withdrawal is the only option. The faster the better.

Or as Simon Jenkins writes about Iraq in The Guardian:

This country has been turned by two of the most powerful and civilised nations on Earth into the most hellish place on Earth. Armies claiming to bring democracy and prosperity have brought bloodshed and a misery worse than under the most ruthless modern dictator. This must be the stupidest paradox in modern history. Neither America nor Britain has the guts to rule Iraq properly, yet they lack the guts to leave.