Eight years and eight months after President George Bush launched operation "shock and awe" in the skies over Baghdad, one of America's wars at last is over. Yesterday, Leon Panetta, the US Defence Secretary, led the ceremony in which, according to military tradition, the flag of US Forces-Iraq was formally retired. By the end of the year, the last American soldier will have left the country, meeting the target date originally set by Mr Bush in 2008, and fulfilled by his successor. Thus the curtain descends on what may be the greatest foreign policy blunder in US history.

In his speech to the troops this week, Commander-in-Chief Barack Obama assured them that their effort had not been in vain, that they were leaving behind an Iraq that was "sovereign, stable and self-reliant", led by a democratically elected government. A better judgement, however, was that offered by then Illinois state senator Obama in 2002, when he denounced the soon-to-begin invasion as "a dumb war, a rash war", based "not on reason but on passion, not on principle but on politics".

Yes, Iraq and the world are better off without Saddam Hussein. But, by any sane cost-benefit analysis, the price paid was far, far too high. That price covers not just the 4,500 US soldiers killed and the 32,000 wounded, as well as at least 100,000 Iraqis (some estimates range five times as high) who perished in the war. On this misguided enterprise, America spent a minimum of $1 trillion, borrowed money that has greatly contributed to the country's runaway deficits.

Then there are the less quantifiable costs: the vast damage to the country's global image inflicted by Abu Ghraib and the appalling behaviour of some US contractors, and the war's unintended consequence of a surge in the regional influence of Iran. Indeed, from start to finish, the US never really understood the country of which it had seized control, as it dismantled the army and other institutions that might have made the transition so much smoother.

Of course, America is not extricating itself entirely. Although Mr Obama failed to reach a deal with Nouri al-Maliki allowing some US troops to stay on to help with training and security, Baghdad will nonetheless boast the largest US embassy on the planet with a total staff of 15,000.

And, although Iraq may have faded from the headlines, between 200 and 300 of its citizens continue to die each month as a result of political violence; far fewer, it is true, than at the height of post-invasion chaos in 2006, but no advertisement for stability or self-reliance either. Nor is there any guarantee that tensions between Shias and Sunnis will not explode after the Americans have departed, once more plunging the country into sectarian strife – or that Iraq will not be thrown into new turmoil by events in neighbouring Syria and/or Iran.

But the outlook is not wholly gloomy. There is hope as well. The Americans' failure to restore Iraq's infrastructure – which was battered by a decade of sanctions before 2003, and then shattered by the war – is a major blot on their record and a reason why ordinary Iraqis will not be sorry to see them go. But Iraq's vast oil wealth means it does at least have the resources to carry out the task itself.

Politically, too, there are grounds for optimism. Iraq, like Iran, is majority Shia, but Mr Maliki has signalled that he will not be Tehran's regional stooge. The domestic security forces are also much improved. One way or another, Iraqis have their best chance of leading safe, normal lives since Saddam came to power in the 1970s. Was the war to get rid of him worth it? Absolutely not. But the dream of the neo-Conservatives who started it, of Iraq as a beacon of prosperity and modernity for the tormented Middle East, might just one day come true.