It is the largest movement of soldiers and materiel in more than four decades, the military said.
By itself, such a withdrawal would be daunting, but it is further complicated by attacks from an insurgency that remains active; the sensitivities of the Iraqi government about a visible U.S. presence; disagreements with the Iraqis about what will be left for them; and consideration for what equipment is urgently needed in Afghanistan.
All the while, the Army must sustain its current force of about 124,000 troops across the country, trucking in fuel, food and other essential supplies while determining what to leave behind for the 50,000 troops who will remain in a mostly advisory role until 2011.
"It's a real Rubik's Cube," Brig. Gen. Paul L. Wentz, the commander of the Army's logistical soldiers, said in an interview at this vast military complex north of Baghdad, which will serve as the command center for the withdrawal effort.
But just as the buildup in the Kuwaiti desert before the 2003 invasion made it plain that the United States was almost certain to go to war, the preparations for withdrawal just as clearly point to the end of the US military role here. Reversing the process, even if the relative stability in Iraq deteriorates into violence, becomes harder every day.
The scale of the withdrawal is staggering. Consider a comparison with the first Persian Gulf War, in 1991: it lasted 1,012 hours, or about six weeks, and when it was over, Lt. Gen. William G. Pagonis, in charge of the Army's logistical operations at the time, wrote a book, "Moving Mountains"' (Harvard Business Press Books, 1992) about the challenges of moving soldiers and equipment in and out of the theater.
He called the undertaking the equivalent of moving the entire population of Alaska, along with their belongings, to the other side of the world "in short order."
The current war in Iraq has lasted more than 57,000 hours, or more than six and a half years. And now Pagonis' son, Col. Gust Pagonis, is one of the leading logisticians assigned to the task of figuring out how to extricate America from the desert.
"When I told my dad what my assignment was, he just laughed and said good luck," Pagonis said.
A substantial reduction in troops is not scheduled to begin until after the January national elections. But preparations for that withdrawal can be seen on the roads across Iraq every night, with an average of 3,500 trucks traversing the nation daily on sustainment and redeployment missions.
The military has largely identified which materials are no longer essential and has begun to move them out of the country, in some cases to Afghanistan. For instance, lumber, ammunition and barriers used to defend against car bombs are all desperately needed in Afghanistan, and as bases are taken apart here, those are among the items sent to the fight there, commanders said.
In August, about 3,000 shipping containers and 2,000 vehicles were shipped out of Iraq, and the heavy lifting is just beginning.
"When the brigade combat teams come out, I want to be in a position where I don't have to deal with the excess equipment and materiel at the same time," Wentz said.
As the pace of withdrawal is stepped up, the US military must also assuage the worries of Iraqi politicians who want the American troops to be less visible, so most missions are carried out in the dark of night.
The Americans hope that by next spring, they will be operating from what is described as a hub-and-spoke system, with six supersize bases and 13 smaller ones. Fewer bases means traveling greater distances, at greater risk.
"The distance between two points does not get any shorter," said Col. Pagonis, asserting that the logisticians in his command - known as "loggies" - are also warriors.
Turning the former US bases over to the Iraqis, and deciding what to give them, have proved to be among the biggest challenges.
Until May, there was no system in place even to figure out who legally owned the property where Americans had set up camp. This led to scenes like the one at Forward Operating Base Warhorse, where a local Iraqi commander showed up essentially demanding a list of items that the Americans were not ready to turn over.
So last spring, panels made up of Iraqi and US officials were set up to help work through some of these issues.
Congress has limited the total value of equipment - like computers, telephones and furniture - that the military can leave to the Iraqis to roughly $15 million per base, but that amount does not include items considered part of the infrastructure, like buildings, sewerage and power plants.
Even coming up with a value for some of the U.S. investments is hard because in many cases the initial costs were inflated by large outlays for security.
Commanders say it is often simply more economical to turn over more equipment to the Iraqis because the cost of moving it is prohibitive.
Last month, the military announced the end of its detention operations at Camp Bucca on the Kuwaiti border and said that $50 million worth of infrastructure and equipment would be given to the Iraqis.
The United States has also brokered a deal with an Iraqi trucking network, led by a coalition of tribal sheiks, to move equipment that is not deemed sensitive between bases. The truckers currently move about 3 percent of all U.S. materiel here, commanders said.
Commanders also said they would closely watch the January elections for what they say about the reliability of Iraq's security forces and the direction the country is heading. But for the planners of the withdrawal, there is no time left to wait and see.
"You can't wait for some big 'Aha!' moment," said Brig. Gen. Heidi Brown, a deputy commander charged with overseeing the withdrawal. "That does not give you flexibility. That just puts you in a box."