In what could prove a critical period for the postwar occupation, most of the 130,000 US troops now in Iraq are scheduled to rotate out over roughly four months starting in January 2004. Most will be replaced by other US troops, but in an effort to draw down the US deployment, tens of thousands may not be.
In either case, armed authority will shift to Iraqis, foreign coalition forces, or incoming troops.
The large-scale handover of security responsibilities coincides with signs of lower morale among many troops and the US public's waning support for keeping troops in Iraq until the country stabilizes. But the transfer injects new risks as the coalition struggles amid guerrilla and terrorist attacks to lay the foundation for a new government. American units familiar with streets, villages, and power brokers of various regions - including Iraq's volatile "Sunni triangle" - will change hands.
Already, violence has flared in two areas where US and coalition forces have recently rotated. To be sure, many factors contribute to such unrest. One of them, however, is the tendency of Iraqi opponents to test newly arrived units less familiar with the territory, military officials say.
In Karbala, a holy city south of Baghdad, conflicts involving Shiite religious factions have escalated and a curfew has been imposed since a Polish-led division took over from the US Marine Corps last month. Last week, three US military police were killed as they tried to negotiate with a group of armed men near one of the city's biggest mosques.
"Transition time is the most vulnerable," the top US ground commander in Iraq, Lt. Gen. Ricardo Sanchez, told reporters at the time of the handover.
Meanwhile, one of the worst friendly fire incidents between US and Iraqi security forces - American soldiers' killing of eight Iraqi policemen near Fallujah last month - coincided with a handover between US military units.
"Any time you get into a situation where units are leaving, there will always be a risk of seams occurring, and the intent is to mitigate those," says Maj. Pete Mitchell, a spokesman for Central Command, which oversees US forces in Iraq. "Does that mean these circumstances are without friction? Absolutely not."
A manpower crunch and morale concerns are among the reasons for the draw-down of US forces, planned to run from January to April as seven large Army units that invaded Iraq last March or rolled in shortly after finish tours of duty. Under the plan, barring a major deterioration in security, the overall number of US troops could fall to 100,000 over the next year, as Iraqi and other coalition forces take over from American soldiers. But some congressmen and military experts have called the desired reductions "wishful thinking."
"Our goal is to increasingly shift the responsibility for Iraqi security to the Iraqis themselves, as is already happening at a pace exceeding by far any recent past experience," Defense Secretary Donald Rumsfeld said Tuesday. He said 85,000 Iraqi security personnel, including police and border guards, now work along with 24,000 non-US foreign troops.
Rotations will be driven by military commanders' assessment of security on the ground rather than a strict timeline, Mr. Rumsfeld stressed. However, many factors are pushing the Pentagon to stay on schedule: low morale among many troops and rising public pressure to bring them home, as well as the complex logistics of deployment and the limited replacements available.
"We've made a commitment to the American service member that he is going to be here for one year.... And that's what we are going to live up to," General Sanchez said earlier this month.
With almost half of its combat units in Iraq, the Army's active-duty component alone cannot sustain current force levels past March if it limits tours to a year and maintains a 3-to-1 ratio of deployed to nondeployed troops, according to a Congressional Budget Office (CBO) report.
Congress has moved recently to boost the Army's ranks. The Senate passed an amendment to the Iraq supplemental funding bill last week that would increase the Army's total manpower, or "end strength," by 10,000. Legislation also passed the House in May that would add 6,000 new positions for high-priority shortfalls in the military as a whole.
Rumsfeld, however, has opposed the Army increase as too costly, and noted that new troops would not be ready for two years.
To meet immediate demands, the Pentagon has mobilized three Army National Guard brigades and plans to call up Reserve support units in coming weeks. It is also considering new Marine Corps deployments, breaking with the past decade's reliance on the Army for peacekeeping and stabilization missions. The rotation will change the mix of US ground forces, with mobile infantry in Humvees and light armored vehicles increasingly replacing tanks and other heavy armored units, commanders say.
Foreign-troop contributions to Iraq could grow with passage of a UN resolution calling for financial and military support of reconstruction. However, a new multinational division may not materialize in time to replace the 101st Airborne, set to leave northern Iraq in February or March.
Meanwhile, handing over security duties to Iraqis following minimal training is risky, experts say. "I'm not sure there's a comfort level across the board that [Iraqi security forces] are ready yet. [US commanders] on the ground are skeptical," says Gordon Rudd, a military historian who returned from Iraq in August after working there for the US-led coalition.