A small but growing number of soldiers who perform credibly in combat turn to high-risk behavior, including drug abuse, drunken driving, motorcycle street-racing, petty crime and domestic violence, once they return home.

As a result, more soldiers are dying by drug overdose, accident, murder and suicide than in combat. Suicide is now the third-leading cause of death for soldiers.

“Simply stated, we are often more dangerous to ourselves than the enemy,” concludes the extraordinary internal Army investigation commissioned by Gen. Peter Chiarelli, the Army’s vice chief of staff.

The study also found that across the Army, leaders have lost visibility and accountability over their soldiers, in many cases unaware that soldiers under their command had abused drugs, committed crimes or even previously tried to commit suicide. Drug testing is done only sporadically, the study found, and there are no central repositories for criminal data.

That same themes are reflected dramatically in the case of five soldiers from the 5^th Stryker Brigade of Fort Lewis, Washington, who are charged with the wanton murder of Afghan civilians in Kandahar last spring. Questions have been raised about how their commanders could have missed such warning signs as drug abuse — some of the soldiers were allegedly smoking hashish in their rooms — that might have led them to look deeper.

No one suggests that such aberrant and ugly crimes can be traced just to the effects of stress. But as Chiarelli acknowledged, the indications of problems within the Army are “troubling.”

And the pressure is unrelenting. Over the next 12 months the Army plans to pull about 66,000 soldiers away from their homes and families and send them into combat in Afghanistan, many for the second or third time. There, these soldiers will replace troops just finishing their 12-month tours.

In all, about 200,000 soldiers will deploy in the coming year in routine rotations to maintain Army forces in South Korea, Kosovo, the Sinai, Iraq and elsewhere. The 45,000 soldiers currently assigned to duty in Iraq are due to be withdrawn by December 2011, unless a revised U.S.-Iraqi agreement enables American trainers and advisers to stay longer, as is likely.

And in Afghanistan, unless President Barack Obama authorizes a major troop reduction next summer, which seems unlikely, planned troop rotations will continue to maintain the 69,000 soldiers in that country. (Roughly 31,000 Marines, Navy and Air Force personnel also serve in Afghanistan.)

Even though some troops have been withdrawn from Iraq, the Army is still straining to fill its overseas commitments.

“The reality is that in the active-duty force, we have very few units in the ‘available’ pool who aren’t heading somewhere,” said Brig. Gen. Peter C. Bayer, director of strategy, plans and policy for the Army operations staff. “It should come as no secret,” he added, “that when you run at the pace we’re on, it comes at a cost.”

Those are mostly hidden behind the Army’s “can-do” ethos and the stoic heroism of its soldiers and families. Desertion and AWOL rates, for instance, are no higher now than during the peacetime years of the1990s, and retention numbers, which measure re-enlistments, are surpassing the Army’s goals.

But behind those simple measures is a darker reality.

Today, more than 100,000 soldiers are on prescribed anti-anxiety medication, and 40,000 are thought by the Army to be using drugs illicitly. Misdemeanor offenses are rising by 5,000 cases a year.

With the pressing need for manpower, the Army has retained more than 25,000 soldiers who would otherwise have been discharged for misbehavior, including 1,000 soldiers with two or more felony convictions.

The most at-risk soldiers are the Army’s youngest, buffeted by leaving home, struggling through basic training, adjusting to their first unit and deploying, all normally within two years. They lack the toughening and resiliency of older soldiers, and often haven’t been in a unit long enough to develop strong ties with other soldiers and leaders.

Among the growing number of Army suicides — which soared past the civilian rate in 2008 and reached a record 239 last year — most are soldiers with less than 24 months in the service. About one third of the Army’s suicides are soldiers who had never deployed even once.

In addition to suicides, the Army recorded 107 fatal accidents among its active-duty soldiers, and 50 murders, an ugly toll of 345 active-duty, non-combat deaths, about 100 more than were killed in combat in 2009.

Much of the stress soldiers endure could be alleviated by time away from combat. But soldiers of all ranks, the Army investigation found, don’t have enough time at home between deployments to recover. “Each time I come back it takes longer to return to what my family and friends regard as normal,” said Bayer, who completed three combat tours in Iraq and now works at the Pentagon. “I’d come home wound tight, and it’s a cumulative effect.”

In fact, it takes 24 to 36 months to return to “normal” from the high intensity of combat, the Army said — while most soldiers are at their home base for 18 months or less between deployments.

“This is uncharted territory,” said Robert Scales, a retired major general, historian, and former commandant of the Army War College. “We have no experiential data to tell us why anything causes emotional collapse after such enormous strain … In some units, it’s not about how many trips to the ‘sand box’ soldiers have made but the emotional wearing that comes from uncertainty and an overbearing sense of foreboding that all too often accompanies units as they deploy multiple times.”

“Frankly,” said Scales, a decorated combat officer who has studied the performance of small infantry units, “I am amazed that the Army and Marine Corps have held together for so long.”

The Army has had an aggressive anti-suicide program under way for some time, and has now begun tackling other problems it has identified. Chief among them: given the hectic pace of training and deployments, commanders often fail to keep track of soldiers who are developing problems and engaging in high-risk behavior. There is no Army-wide database for drunken-driving citations or misdemeanor offenses, for instance.

The Army has tightened its screening of recruits to weed out those with fragile personalities. But already, 75 percent of American teenagers aren’t eligible for the Army because they are overweight or have other physical disabilities, can’t pass the entrance exam, or they have a criminal record. And Army recruiters are prevented by law from asking for recruits’ medical records.

Senior officers acknowledge that all these problems are likely to haunt the Army as long as the pace of deployments remains so high.

“The good news,” Chiarelli observed this summer, “is that soldiers are seeking behavioral health care in record numbers.”