Last US troops withdraw from Iraq
Earlier this week, standing next to Prime Minister Nouri Maliki, President Barack Obama left to history the verdict on the wisdom of the 2003 invasion of Iraq.
He could have quoted his predecessor, George W Bush, who described it in 2004 as a "catastrophic success".
As the United States closes the book on Iraq, was it worth it?
The short answer is that it is too early to tell, but probably not.
At a purely tactical level, we know a lot already. Despite regular but greatly reduced violence, Iraq is reasonably stable.
The most notable achievement is the creation of a democratic (if less than optimal) government in Baghdad, although we in the United States are in no position to throw stones about gridlock.
The Arab Spring has brought forward a different approach to political transformation, supporting indigenous movements with limited military support (Libya) rather than imposing change through invasion and extended occupation (Iraq).
Whether one works better than the other, we'll have to wait and see.
If there is real competition among all Iraqi factions and a peaceful transition of power, this does qualify as a real accomplishment”
In response to the Arab Spring, Iraqi Prime Minister Nouri Maliki announced he would not run for re-election in 2014.
If there is real competition among all Iraqi factions and a peaceful transition of power, this does qualify as a real accomplishment.
But all of this came at a tremendous cost, in terms of Iraqi lives, American lives, national treasure and lost stature both in the region and around the world.Strategic miscalculation
The Bush administration underestimated the challenge and failed to send adequate forces, both military and civilian, to protect the population and prevent the multiple insurgencies that developed.
It eventually got it right, but not before severe damage was done.
The direct cost of transforming Iraq will top $1tn (£649bn). That is a staggering sum, particularly in contrast to the few hundred million being invested today in advancing democratic transitions in Egypt, Tunisia and Libya.
The indirect costs are severe as well, especially the stress and strain on a volunteer military that is suffering from high rates of suicide and post-traumatic disorder.
Better protective gear saved many lives during a conflict that introduced a global weapon of choice - the improvised explosive device. These soldiers, many missing limbs and normal motor functions, will require a lifetime of care.
The focus on Iraq in the aftermath of 9/11 made perfect sense. Osama Bin Laden's narrative centred on the US military's ongoing containment operations in the Gulf, which the late Saudi militant viewed as an occupation of sacred lands, and long-standing support for dictatorial regimes.
The status quo was leaking and fraying badly.
Moving the region towards political reform, better governance and eventually democracy was consistent with US long-term interests and values and the right prescription for al Qaeda's violent political extremism.
But rushing into Iraq was a huge strategic miscalculation.
The Bush administration thought that the invasion of Iraq would offer a "demonstration effect" that could transform the Middle East. It did, but not in ways the Bush administration intended.
Having been defeated in Afghanistan, Iraq provided al-Qaeda with a new battlefield to establish a beachhead in the heart of the Middle East.
Iraq validated Bin Laden's narrative that the United States was at war with Islam. Wannabe holy warriors flocked to Iraq from across the Islamic world to join in jihad. The anger was manifest in ripple attacks in London, Madrid and elsewhere.
The tide of public opinion turned against al-Qaeda in 2007, primarily because jihad was killing far more Muslims than Westerners.
But al-Qaeda's loss is not America's gain. It will take years to repair US standing in the Middle East.Message to Tehran
The clear winner following the Iraq invasion was Iran. Without Iraq as a counterweight, Iran is far more assertive now, even though the potential transformation of the region has put Iran on the defensive.
The fall of Saddam (and Muammar Gaddafi in Libya) drove home an inescapable message to Tehran: develop a nuclear capability as soon as possible and never give it up”
Iraq is not now an Iranian client state, but the Iraqi prime minister's reluctance to intervene in Syria demonstrates that the fragile Maliki government is going to pick its battles with neighbours carefully, leaving room for Iranian mischief at US expense.
The fall of Saddam (and Muammar Gaddafi in Libya) drove home an inescapable message to Tehran: develop a nuclear capability as soon as possible and never give it up.
If the nightmare post-9/11 scenario is nuclear know-how in the hands of a terrorist organisation, that challenge is ironically more complex with a resurgent Iran than it was with an inscrutable but contained Iraq.
The international community could pull a diplomatic rabbit out of the hat and convince Iran to comply with its obligations under the Non-Proliferation Treaty (which allows for civilian nuclear power, but not a weapon).
It is far more likely Iran maintains its current course and figures out how to build a bomb (whether or not it actually does so).
A pre-emptive attack delays this outcome, but does not prevent it.
There is a real possibility of a nuclear arms race involving Saudi Arabia, perhaps Turkey and Egypt, and potentially others. The more nuclear stuff in circulation, the greater the risk of proliferation by a non-state actor, the overarching concern that led the Bush administration into Iraq in 2003.'Second wave' fear
But how did we get from al-Qaeda's attack on the World Trade Center and the Pentagon to Baghdad and the vain search for weapons of mass destruction?
Despite the efforts of Vice-President Dick Cheney to join the dots between Iraq and 9/11 that simply did not exist, there was no plausible alliance between Saddam and Bin Laden.
Al-Qaeda's vision of a caliphate, impractical as it might have been, came at Saddam Hussein's expense.
The enemy of an enemy can become an ally at any moment in time, but there is a difference between giving a terrorist group financial or technical support (nothing close to that provided by Iran or even Pakistan, which has been perversely rewarded despite its known association with terrorists), and giving them an existential weapon that can be turned against any state and any apostate leader.
Saddam Hussein was never going to give Bin Laden a bomb.
Nonetheless, the Bush administration's sense of urgency came from the combination of two surprise attacks, not just September 11 but especially the anthrax letters one month later.
As former National Security Advisor Secretary of State Condoleezza Rice writes in her recent memoir, Iraq came up at a Camp David meeting right after 9/11 and was dismissed as a "distraction."
But after the anthrax letters reached the US Capitol, terrorism and weapons of mass destruction merged within the same frame.
"We were all convinced it was al-Qaeda's second wave," Ms Rice recalls.
Having declared there would be no distinction between the actual terrorists and those who support them, calculations shifted.
The unknown trumped the known, and the push to Baghdad was underway.
The rest, as they say, is history.