"I think we're going to find that this president, by not putting in place a status-of-forces agreement with the Iraqi leadership, has pulled our troops out in a precipitous way and we should have left 10-, 20-, 30-thousand personnel there to help transition to the Iraqi's own military capabilities," Romney said in an interview with Fox News Sunday.

Romney comments -- his first appearance on a Sunday talk show in more than two years -- airs after the last US troops crossed out of Iraq into Kuwait, nearly nine years after a US invasion that resulted in the deaths of 4,474 US troops and tens of thousands of Iraqis.

US military commanders advocated leaving a residual US force in the country to train and support the Iraqi military, but Prime Minister Nuri al-Maliki refused to extend legal protections to US troops under a status of forces agreement, scotching that option.

Among the unresolved questions raised by the US departure is the degree of Iranian influence over Iraq's Shiite dominated government and how sectarian tensions will play out.

"I'm very concerned in this setting. I hope it works out," Romney said.

Romney has been an on-again, off-again leader in the Republican fight to nominate a candidate to go up against Obama in the November 2012 presidential elections.

Former US envoy to China Jon Huntsman, one of Romney's rivals for the party's presidential nomination and the Republican candidate with the strongest foreign policy credentials, also assailed the withdrawal.

"President Obama's inability to reach a security agreement in Iraq is a product of the administration's failures in the region," said Huntsman spokesman Tim Miller.

"Governor Huntsman would have supported an agreement that left a small troop presence that could have assisted with the training of Iraqi security forces and vital counter-terror efforts," Miller said.

+Human cost

Since the US invasion in March 2003, at least 126,000 Iraqi civilians were killed in the war, according to Boston University professor Neta Crawford. In addition, another 20,000 Iraqi soldiers and police were killed, along with more than 19,000 insurgents. British group IraqBodyCount.org puts the number of documented Iraqi civilian deaths from violence at 104,035 to 113,680.

For the US-led coalition, the Pentagon says the United States lost 4,474 troops, of which 3,518 died in combat. This figure is by far the highest of an invading coalition country. Britain was next, with 179 troops killed, according to the Defense Ministry. Nearly 32,000 American troops were also wounded.

In November, 187 Iraqis were killed by violence, including 112 civilians, 42 policemen and 33 soldiers. This figure compares to 2,087 people killed in January 2007. By comparison, 2,045 people were killed in the first nine months of 2011. These are all according to figures released monthly by the Iraqi ministries of health, interior and defense.

And the United Nations estimates that 1.75 million Iraqis were made refugees by the war, forced to flee to neighboring countries or to displace their families to other parts of the country.

+Troops deployed

At the start of Operation Iraqi Freedom, about 150,000 US troops were stationed in Iraq, supported by 120,000 forces operating outside of the country. Roughly 40,000 British troops were deployed as well during the course of the war.

The US troop presence reached 165,000 at the end of 2006 before President George W. Bush decided on a "surge" of 30,000 reinforcements in a bid to counter spiraling violence.

In September 2010, the US combat mission officially ended and 50,000 American troops remained on the ground to advise and train Iraqi forces as part of the newly dubbed "Operation New Dawn." The last of those US troops have now left Iraq.

+Financial cost

The Pentagon has spent nearly $770 billion since 2003 on operations in Iraq. Categorized as overseas contingency operations, the sum is treated separately from the main defense budget, which has also included some funds for the Iraq war.

The World Bank estimates that Iraq's GDP fell by 41 percent in 2003.

The Iraq war and reconstruction is also projected to have cost US taxpayers $256 million per day from 2003 to 2012, according to the Congressional Budget Office.

Any accounting of the war's price tag also has to include billions in US civilian aid to Iraq, as well as the cost of care provided to wounded soldiers and veterans.

US government statistics do not distinguish between veterans who served in Iraq or Afghanistan, as a large number of the 1.25 million veterans were deployed to both wars.

By the end of 2010, the United States had already spent nearly $32 billion on medical treatment for wounded troops and payments for disability pensions, a benefit veterans receive for life.

The future cost of medical care and pensions for veterans will grow exponentially in coming decades. Linda Bilmes, professor at Harvard University, estimates that pensions through 2055 for veterans will reach $346 billion to $469 billion, mainly due to health care costs.

+Other losses

Around 60 percent of the Iraqi National Archives, equivalent to tens of millions of documents, went missing, were damaged or were destroyed as a result of water leaks and a fire at a storage center in the aftermath of the 2003 invasion, according to INA director Saad Iskander.