WASHINGTON -- Nearly a third of the 1 million US military personnel called to duty in Afghanistan and Iraq have served two or more extended tours in combat zones, according to figures compiled by the Defense Department.
The breakdown indicates that of the 955,609 members in the armed forces, including active-duty and reserve personnel, who have been deployed for operations in Afghanistan or the Persian Gulf region since the Sept. 11, 2001, terrorist attacks, 303,987 have been sent overseas more than once.
The data, as of Sept. 30, demonstrate the extent to which the Afghanistan operation and larger Iraq mission have placed enormous strains on soldiers and their families and how the frequent deployments are threatening the Pentagon's ability to retain veteran soldiers in the future, according to military officials and specialists.
''Our research indicates that deployment is a big influence on people's commitments to military service," said Harold Weiss, a psychology professor and codirector of the Military Family Research Institute, a government-funded center at Purdue University that is conducting a study on how military deployments affect families.
''Both spouses and members are part of the decision-making process when a family decides to stay in the military," he added. ''It's a family decision because the military is not a job; it is a life. Multiple deployments will make it harder to stay in the military."
The Pentagon's longtime goal has been to deploy individual soldiers overseas only once in a four- or five-year period so that the troops and their families can fully prepare and adjust for extended duty away from their home base.
But the war on terrorism, along with the US-led invasion of Iraq, has stretched the military so much -- ground forces, in particular -- that the Pentagon cannot predict when it will again be able to stagger deployments over several years.
There are ominous signs that the strain on the force is hurting the military's ability to fill the ranks. The Army National Guard, which has tens of thousands of soldiers deployed overseas, saw its recruiting numbers slide by 30 percent in October, part of a trend that began last year, the first year of the Iraq war.
Many Afghanistan and Iraq veterans say the frequent periods away from home raise new concerns about how their families will be able to cope while they are away and how they will be compensated afterward.
Some worry they may be forced to leave military service if they are not confident that the Pentagon is doing everything possible to look after them and their families.
''When we joined, we understood we were not taking a job at a department store," said Lieutenant Colonel Samuel Poulton, an Army Reservist from Chelmsford, Mass., who served in Iraq and Kuwait last year at the same time that his 27-year-old son, an Army staff sergeant, was deployed to the Navy's detention facility at Guantanamo Bay, Cuba.
''We went into this with our eyes open, but the concern is how will we and our families be treated?" added Poulton, 56, who is a sociology teacher at Waltham High School. ''Will we have healthcare, educational opportunities? Will our families be taken care of if something bad happens?"
The Pentagon figures provide the clearest picture yet of how many military personnel have served overseas since the Sept. 11 attacks and how often. The data include full-time troops in the Army, Marine Corps, Navy, Air Force, and Coast Guard, as well as reserve forces and the Army and Air National Guard.
The Army and Marine Corps have carried much of the load. Of about 500,000 members in the active Army, more than half -- 279,393 -- have been sent overseas in the past three years. And of those, 34.6 percent have served multiple tours, some for a year or more and others several months at a time.
For the smaller Marine Corps, the percentage of the total force dispatched to Afghanistan or the Persian Gulf is greater -- 98,979 of about 120,000. Of those, 27.6 percent have done multiple tours, according to the Pentagon's count. The corps will be adding about 3,000 more troops to reduce the burden.
For part-time soldiers who leave jobs as well as families behind, the percentage serving multiple tours is even higher. Of the 90,649 Army National Guard soldiers deployed, 35.9 percent have been called up more than once.
For the Army Reserve, 34.6 percent of the 64,978 that have served since the Sept. 11 attacks have returned home, only to be redeployed within months.
Meanwhile, nearly half of the 41,093 members of the Air National Guard called to active duty have served two or more tours in the same period.
Military specialists fear that the full impact of the frequent deployments has yet to be felt and that the outlook could get worse, especially if another major military challenge arose that would require large numbers of armed forces.
''The full impact of being overstretched hasn't hit us yet because we haven't encountered another problem," said Loren Thompson, a military specialist at the conservative Lexington Institute in Arlington, Va. ''Afghanistan and Iraq are not the full range of challenges that could be ahead, but require most of our capabilities."
He said the 10 divisions of the active-duty Army ''clearly can't hack it, so we are calling up the reserves. It is a debacle."
Others worry that the Pentagon's support system is not up to the task of preparing soldiers and their families for extended tours as well as meeting their financial and psychological needs during and after the deployments.
''The broad snapshot of family support gained through research shows that many programs and services are in place to help military families and are doing so, but on an inconsistent basis," the National Military Family Association reported last summer. ''Families must have a comprehensive and responsive support system to prepare and sustain them. Families serving on the home front remain resilient. However, as the high pace of military operations continues, their strength wears down."
A recent study by the federally funded RAND Corp. warned that the frequency of deployments could be a looming problem.
''If deployment adversely affects retention, the military may face manning shortfalls, higher personnel turnover, and difficulties maintaining readiness," according to the report, entitled ''How Does Deployment Affect Retention of Military Personnel?"
''You have a tension between the needs of the war effort and the military community trying their best to provide support to military families," added Weiss, the military researcher at Purdue.
''We are trying to understand what support the military can provide while they are away and when they are reunited," so that officers and enlisted personnel do not start leaving in droves because of strain on their families.
Poulton, the reservist who serves in the 309th Combat Support Hospital based at Hanscom Air Force Base, said he and his son's concurrent tours last year were ''a terrible burden for my wife. She is a real hero."
And their work is probably not over. ''I think that all of us expect a second deployment before the Iraq operation has been completed," he added.
Bryan Bender can be reached at email@example.com.