The U.S. military has begun its new year rotation of forces into Iraq which is one of the biggest movements of American troops in decades.
Over the next four months, over 240,000 U.S. soldiers and marines and their equipment will be moving into and out of Iraq.
The rotation will bring relief to the war-weary soldiers who first moved into Iraq early last year to topple Saddam Hussein. Since then, these same combat veterans have struggled non-stop against insurgent supporters of the deposed regime, a struggle that has seen more U.S. troops lose their lives since major combat ended than during the actual war.
The replacement of these battle-tested forces with fresh troops is a logistical challenge. General Richard Myers, chairman of the military's Joint Chiefs of Staff, admits there will be what he terms turbulence.
"In the next four months, we're going to pull off a logistics feat that'll rival any in history, I think, as we move a major part of the Army," he said. "Well over the majority of the Army combat units and a lot of the reserve component will move - those in Iraq [will go] home, and those here in the United States and other places in the world [will go] to Iraq or to Afghanistan. It's going to be a very big project that we've been planning for a very long time. So there's going to be a lot of turbulence in the system, as you would expect."
But the rotation also poses an enormous security challenge. Experienced troops with knowledge of Iraq's geography, its political dynamics and its people are leaving.
Defense Secretary Donald Rumsfeld says that represents a real loss. But he notes there will be an overlap, so outgoing commanders and soldiers can effectively share their experience with the replacement forces.
"The people going over are ready, but the people there are experienced and really know their stuff," he said. "And who would you rather have there? So what we're going to have to do is to manage that transition very carefully. There's going to have to be overlap. We're going to have to be sensitive to the fact that the knowledge that's built up there and the relationships have to be transferred and they have to be transferred in a manner that's appropriate."
Military officials say the troops going into Iraq will be better prepared for the kinds of duties and operations now required in a post-war climate.
The new forces will include more military police and civil affairs specialists. The new troops will also be more mobile and less heavily-armed. There will be fewer tanks and other heavy armored vehicles, for example.
The Army also plans to deploy more unmanned aerial reconnaissance drones in an effort to boost intelligence gathering on Iraqi insurgents.
In addition, a special team has already been sent into Iraq in an effort to improve the military's ability to counter the bombs and other improvised explosive devices being planted by the insurgents.
Those devices have been responsible for the deaths of scores of American soldiers in recent weeks.