WASHINGTON -- The majority of US troops scheduled to rotate into Iraq next year already have served at least one tour of duty, making them one of the most experienced combat forces in recent history, according to Pentagon officials and a review of deployment figures.
But as the tempo of deployments continues at a high pace, military specialists are also increasingly concerned about the effect the prolonged overseas duty could have on the troops and their families, as well as on the military's ability to quickly respond to other trouble spots.
At least two-thirds of the 92,000 Army troops set to deploy to Iraq beginning in 2006 will be returning for the second time since the US invasion nearly three years ago, the Army predicts. The soldiers will constitute the bulk of American forces that are expected to remain in Iraq until at least 2008.
One New York-based Army unit selected to redeploy just returned from a yearlong tour in July. An Army brigade, headquartered in Germany, came back in February. Another Army brigade in Hawaii on tap to return to Iraq completed a long deployment to Afghanistan earlier this year. For some soldiers who have transferred to new units, their upcoming deployment will be their third since the US-led invasion.
Marine Corps units and replacements from other military branches have yet to be identified.
The unique skills and knowledge of the returning service members, ranging from counterinsurgency tactics to firsthand knowledge of Muslim customs and culture, will pay significant dividends, top military officers say. But with the US presence in Iraq expected to last at least several more years, the increasingly frequent deployments also bring new worries about the mounting toll on an all-volunteer force.
''You get a very experienced force, especially at the command level. And that is a very positive thing," said retired Army colonel Michael Doubler, a Vietnam veteran and military historian. ''But it puts a lot of strain on the families. How much strain can they take? We don't know that, but we might be getting ready to find out."
Lieutenant General James J. Lovelace, the Army's director of operations, estimates that 75 percent of the active-duty Army of about 520,00 troops has served at least one overseas tour since September 2001. Of those, an estimated 100,000 have already served two or more tours during the same period.
''This is one of the most seasoned combat forces the Army has fielded since Vietnam," said Colonel Joseph Curtin, an Army spokesman.
The system for rotating troops into Iraq and Afghanistan is far different from the situation during the Vietnam War, when US combat forces were last engaged for an extended period. At that time, with the draft in place, the Pentagon could call up new recruits as they were needed.
Today, the Pentagon is relying on a finite number of men and women to meet its commitments. Moreover, units that fought in Vietnam were permanently stationed in Southeast Asia, and fresh troops were rotated into those units. In Iraq, units train and deploy for up to a year and then return to their home bases.
According to the latest deployment statistics from all the branches, more than 1.2 million active-duty, reserve, and National Guard troops have been deployed in Operation Iraqi Freedom or Operation Enduring Freedom in Afghanistan. Of those, more than 871,000 have served one tour and about 316,000 have served more than one.
Commanders highlight how the trend of multiple deployments makes good military sense. They contend that the troops now preparing for Iraq duty are well prepared for the physically and mentally grueling tasks of battling the insurgency and training Iraqi forces.
The 10th Mountain Division's Second Brigade has been home only four months and was told this week it is going back to Iraq.
''The Second Brigade is a battle-hardened, highly trained, and uniquely qualified unit with previous experience in Iraq," said Major General Benjamin C. Freakley, commander of the division, based in Fort Drum, N.Y.
Officials estimate that nearly all of the Second Brigade will be returning with their unit when they deploy to Iraq next year.
Other units, including those from the Army's First Infantry Division, are in a similar situation.
''The division returned from a yearlong deployment to Iraq in February 2005," said Major Bill Coppernoll, a spokesman for the division, which is based in Schweinfurt, Germany, and will be redeploying its Second Brigade next year. ''We have had a large turnover of personnel since then, but there still are quite a few soldiers that are still here and will be returning a second time."
The benefits of having such a seasoned force are high, according to specialists.
''The downside to this is well understood, but there is an upside to this as well," said John Pike, a military analyst at GlobalSecurity.org in Alexandria, Va. ''You have combat veterans who have rested up. They have seen Iraq, they have taken its measure, there is not as much learning for them to do."
Still, others caution that the strains could soon become too heavy to bear for some troops and their families.
The Pentagon ''needs do a better job of planning so that they can adequately provide for our troops and their families," said US Representative Martin Meehan, a Lowell Democrat and member of the House Armed Services Committee.
Retired Army General George Joulwan, former commander of NATO and of all US military operations in Europe and North Africa, said: ''It is easy to say that is what they get paid for. But I worry that we have a volunteer military that makes up less than 1 percent of the population and is constantly providing security for the other 99 percent."