KANSAS CITY, Mo. — American soldiers in Afghanistan aren't supposed to drink booze. They're not supposed to stray "outside the wire" of their fortifications on their own. And they aren't supposed to train their rifles on innocents.

KANSAS CITY, Mo. — American soldiers in Afghanistan aren't supposed to drink booze. They're not supposed to stray "outside the wire" of their fortifications on their own. And they aren't supposed to train their rifles on innocents.

Yet that's just what Army Staff Sgt. Robert Bales is accused of: drinking and then slipping out of his Kandahar province outpost on March 11 and shooting to death 17 civilians — nine of them children — in their sleep.

A country numb to the tragic frequency of suicide among combat veterans and climbing divorce rates among military families now must take notice of atrocities.

"One country can't keep an occupying force in another country for 10 years and not expect for things now and again to go wrong," said Cindy Williams, a defense analyst and personnel specialist at the Massachusetts Institute of Technology.

Run enough troops through enough bloody combat tours, said Williams, and "you can't expect otherwise. That's the problem."

Bales, locked up at Fort Leavenworth and facing a possible death sentence, already is being offered as evidence of a nation long at war.

None but the most callous would excuse such an act as Bales was formally charged with Friday. But a growing chorus of analysts sees an explanation: The peacetime-sized U.S. military, and the Army in particular, has been pushed through the combat meat grinder long enough that the unspeakable becomes the unavoidable. When Iraq veteran Valerie Brown heard that an American soldier had been accused of slaughtering Afghan villagers, her first reaction was, "Oh, that poor man's family."

"My second reaction was, 'How many times had he been over there?' " the former soldier and helicopter crew chief said. "No, I'm not justifying it. I'm not saying it wasn't wrong. I'm just saying, 'How many times?' "

Civilian deaths, "collateral damage" in Pentagonspeak, have been an Afghan way of life for generations pre-dating the Soviet invasion of the country in 1979. The United Nations said civilian casualties there have climbed each of the past five years.

More than 3,000 died last year, according to its accounting, more than 3 out of 4 at the hands of the Taliban and other insurgents. U.S. deaths accounted for 418 of the 566 NATO fatalities last year.

It's a country still peppered with landmines that blow off the legs of shepherd boys, where assassinations of officials are carried out routinely from passing motorcyclists and where mourners at funerals are blown apart by suicide bombers. Just last, 13 Afghan civilians, including nine children, died when their vehicle hit a roadside bomb in Uruzgan province.

But when it is NATO bombers that mistakenly obliterate a convoy of wedding revelers, many Afghans choose to believe it was by design, not a tragic mistake. Similarly, the Pentagon's insistence that the recent killings were the work of a single, rogue soldier is widely questioned there. Even President Hamid Karzai, often belittled in the region as an American puppet, went so far as to demand the U.S. withdraw from villages across the country after the massacre. He called foreign troops "demons."

"Death and violent death happen there regularly," said John Nagl, a professor at the U.S. Naval Academy and a chief author of the U.S. military's most recent rewrite of counterinsurgency doctrine.

He noted the Afghans are hardened by decades of warfare and death. They are more likely to riot against what a largely tribal culture interprets as direct insults to their faith and sensibilities by the wealthy, powerful and infidel West.

Early this year video emerged of U.S. troops urinating on corpses of enemy Afghan fighters. Then came the accidental burning of Qurans on an American base. Many Americans see the urination episode as an understandable reaction of young troops under stress, the Quran incident at worst as a sloppy cultural mistake. To Afghans, though, those incidents are profound.

"What they hold on to is the Quran," Nagl said. "To have their ally intentionally commit sacrilege is what really gets them angry." Six Americans consequently were murdered by their infuriated allies.

Now, the March 11 slaughter is yet another reason to resent the American presence and to question its objectives. And that makes the prosecution of Bales that much more sensitive.

Afghans are outraged that he was whisked out of their country, beyond the reach of Islamic Shariah law.

Defense Secretary Leon Panetta, certainly wise to perceptions in Afghanistan and the enormity of the allegations, has said the death penalty "could be a consideration."

Bales' removal from Afghanistan comes from a long-standing "status of forces" agreement between Washington and Kabul that protects occupying troops from local prosecution. It was U.S. insistence on such conditions and Iraqi pride that contributed to the speeded-up withdrawal of American forces from that country.

His case also could provide another test on which Americans and the rest of the world could choose to judge how the country holds its troops accountable for crime in a war zone.

The U.S. does not have a record of delivering severe penalties in some of the highest profile war crimes.

The My Lai massacre of 500 unarmed villagers in Vietnam in 1968 was initially covered up by high-ranking officers. Just one lieutenant, William Calley, was convicted in the case. Although he was sentenced to life in prison, he ultimately spent just three years under house arrest. From 2001 through 2011, the Army prosecuted 44 soldiers for killings in combat zones. Of those, 30 were convicted on homicide charges, six were convicted of lesser offenses and eight were acquitted.

In Iraq, eight Marines were implicated in the killing of two dozen unarmed people in Haditha in late 2005. Just one now faces jail time, and no more than three months. In Fallujah the year before, another high-profile case saw two Marines charged with gunning down four unarmed civilians. One was acquitted in a civilian trial; the other pleaded guilty to dereliction of duty with no jail time.

Eleven of 12 soldiers belonging to a rogue "kill team" were convicted in the murders of three Afghan civilians in Kandahar province in 2010. They came from the same U.S. base as Bales, Joint Base Lewis-McChord in Washington state.

Bales' case is sure to get filled in with details and disputes that will muddy a cause-and-effect between the killings and his personal history. Yet the broad strokes of his time in the Army make for a handy microcosm.

There's a growing expectation that his defense attorneys will make a case that he's a victim here, too. His trip to Afghanistan was his fourth run through combat zones since enlisting in the Army shortly after the Sept. 11, 2001, terrorist attacks.

He'd seen his share of death. He'd lost friends to bombs and bullets. Bales suffered two serious injuries in Iraq. One was head trauma related to a roadside blast that hit his vehicle and the second that cost him part of a foot.

Many expect his lawyer to claim post-traumatic stress disorder and perhaps traumatic brain injury — the two signature wounds of modern warfare.

A study by the RAND Corp. military think tank estimated that 350,000 people who served in Iraq and Afghanistan have mental health problems. In 2009, the Defense Business Board released a study describing soldiers who had served more than 25 months in combat zones as "over stressed." That was three years ago, and it accounted for upward of 8 percent of all enlisted personnel.

The Army tallied 166 suicides last year among active-duty troops and another 116 not on active duty. Divorce rates climbed 40 percent among military couples between 2001 and 2011. A UCLA psychology professor who studies the phenomenon expects that statistic to get worse.

"It's hard to maintain a relationship when one person has been damaged," Benjamin Karney said.

To be sure, millions of other troops survived the war on terror with their wits fully about them. While experts say continued deployments strain troops and their families, only the smallest fraction becomes so unstable to pose a violent threat. The U.S. military, meanwhile, still has a level of discipline and professionalism far more pronounced than the forces it meets in combat.

The wars of this era, however, have tested American military stamina in different ways than those of any time before. Close to 800,000 American troops have done not just one long year in a combat zone, but two or more tours in Iraq or Afghanistan. Some were kept in service through "stop loss" provisions that extended their duty involuntarily. Some were sent overseas ostensibly for 12-month stretches that were extended to 15 months.

"The fact that somebody would snap was a real possibility," said John Allen Williams, a military studies professor at Loyola University in Chicago. "We have a military that has been overstressed by two wars."

Tens of thousands of soldiers remain in the service but ineligible for more deployments while the Army sorts out whether the strain and injuries they've endured make them unfit for combat.

In fact, the defining characteristic of American warfare in this century is the way it has asked so much for so long of so few.

"The strategic assumption when the war began is that it would be a blitzkrieg," said Donald Abenheim, a research fellow at the Hoover Institution on War, Revolution and Peace.

Instead, a small volunteer force has endured unprecedented multiple deployments. In previous conflicts, the draft mobilized a larger force. That, in turn, protected soldiers from years and years of tours. Most in Vietnam, where the ranks were swelled by a military draft, served a single tour. World War II lasted just five years.

Combat was also different, Abenheim said. The actual lines of war were decisive and the enemy was clearer.

Today, troops feed villagers and share tea one minute, he said, "the next minute you're terrified the very same people are going to blow your brains out. That leads to a very distinct mentality and a very distinct approach."

It also helps explain why not everyone is eager to vilify Bales, Abenheim said.

The sergeant is "every man and yet a loser and a mass murderer and yet a hero. People can respond to his biography "… people are sick and tired of all these wars," he said.

"That doesn't absolve this man. You're still not supposed to go out and slaughter children."