Under the cover of dark, the 4th Stryker Brigade, 2nd Infantry Division, crossed the border into neighbouring Kuwait ahead of the planned declaration of an end to US combat operations in Iraq by an August 31 deadline.
The pullout came two days after a suicide bomber killed 59 people at a Baghdad army recruitment centre in Iraq's deadliest attack this year, sparking concern the country's forces are incapable of handling security on their own.
"Yes, they did," Lieutenant Colonel Eric Bloom told AFP, when asked if the brigade had crossed into Kuwait. "The last one crossed at about 6:00 am this morning."
"They have a few more days to clean the equipment, prepare the equipment, get it ready for shipment and then they'll fly out (to the United States)."
It took two days for 360 vehicles and 1,200 soldiers to travel from Camp Liberty on Baghdad's outskirts and Camp Taji north of the capital, through the Shiite south, and into the Gulf emirate, Bloom said.
He said the rest of the 4,000-strong brigade departed the country by air.
Captain Russell Varnado at Camp Arifjan, a major US base about 70 kilometres south of Kuwait City, told AFP "the combat troops have finished moving."
"The troops are transitioning now. They are scheduled to go back home soon," he said, without giving a specific date.
Kuwait, which hosts several American military camps in its northern desert close to the Iraqi border as well as a naval base, was used as the launch pad for the 2003 invasion.
About 56,000 US soldiers remain stationed in Iraq, with that figure set to drop to 50,000 by September 1, less than a third of the peak level during "the surge" of 2007.
At that point, the US mission in Iraq will be re-christened "Operation New Dawn", from "Operation Iraqi Freedom" -- the name given to American operations here since the invasion.