By Nick Childs
BBC Pentagon correspondent
The US has begun rotating its forces in Iraq in one of the largest movements of American troops since World War II.
Iraq is leaving US forces stretched
About 250,000 military personnel will move in and of the country over the next four months.
Some 125,000 American troops in Iraq will be replaced by a 110,000-strong, less heavily armed force, including more National Guard and Reserve units.
Troops are also being replaced in Kuwait and Afghanistan, where 11,000 soldiers are on active duty.
The return home this week of the first couple of hundred troops of the 101st Airborne Division, and the departure of new forces from the 82nd Airborne, mark just the early stages of a massive overhaul of US forces in Iraq.
In the next four months, the US troops in the country now will be replaced by a similar number of fresh soldiers and marines.
Similar changeovers are occurring in Kuwait and Afghanistan in what the Pentagon believes is the largest such movement of US forces in decades.
The wholesale exchange of units is very different from the way the American military sustained its Vietnam operation - constantly drafting troops in and out on an individual basis.
It is a reflection of the huge capabilities of the US military, but also a reminder of the strains the US army in particular is being put under by the Iraq operation.
Threat of attack
US commanders believe the troop rotation will leave them with a more flexible force better equipped to deal with the kinds of threats now faced in Iraq.
But they also acknowledge that the changeover could be a time of heightened vulnerability, as battle-hardened troops are replaced by fresh ones.
So there will be a deliberate overlap and staggering of the handovers.
New teams of experts are being sent to help deal with threats to convoys like roadside bombs.
And commanders are concerned that the Iraqi resistance forces could try to exploit this period of change.
But the Pentagon still believes this is the best way to maintain its military presence in Iraq.