This was a preventive war designed to prevent Iraq from using or proliferating its presumed weapons of mass destruction to terrorists or terrorist states, but those weapons did not exist. Ironically, if they had, we did not even send enough troops into Iraq in the first place to properly secure the supposed weapons sites.
We fought a far longer war and far costlier war than we ever could have imagined.
This has been enormously expensive both in terms of lives and treasure. If we had known in 2003 what it was going to cost and what we were going to get for that cost we certainly wouldn't have gone into Iraq. It's reminder of the extraordinary risks one takes when one chooses to roll the iron dice and that it is impossible to predict the long-term results of war.
For five years Iraq was the most important item on policy-makers' agenda. That meant we allowed China to steal a march on the United States. It gained economically, militarily and perhaps even diplomatically as the United States demonstrated it was not the unquestionable superpower that many believed it was at the start of 2000s.
Britain chose to be a good ally to the US but paid a much higher price than she could ever have expected. The special relationship is recovering, but it was damaged, as were many other American alliances. This is not going to go down in history as one of the best decisions of either Her Majesty's government or the American government.
There were grave questions in the Pentagon about the decision to invade, and I was among those who opposed the invasion. I thought Iraq had weapons of mass destruction but I thought it could be deterred – that was after all how we dealt with the Soviet Union, which unquestionably had such weapons.
But once we invaded without a plan for what came after Saddam fell we had a responsibility to provide stability and security.
We learned slowly – too slowly – a number of bitter lessons about how to stabilise a country affected by insurgency and in 2006 during the full scale civil war between Sunni and Shia it looked like the war was going to be lost.
But we changed tactics, George W Bush made a courageous decision that almost no one agreed with to deploy the surge forces and we had a far better outcome than we could have hoped for than in 2007 when Gen David Petraeus took command.
There are glimmers of light in this story. Battlefield medicine has improved dramatically. We have learned lessons about integrating economic development and good governance with military force. We have become the most capable counter-insurgency force in history.
But the benefits of the war are questionable and indirect, and the impact of what has happened there won't be clear for at least a decade.
We are leaving an Iraq that is the semblance of a democracy and that has the ability to take care of itself. The good news is that it is a very rich country and is well able to afford what it will need. But is going to need a lot of help for some years.