Bad things happen when a nation tries to fight too many wars with too few troops. The blunt fact about the U.S. army and Marines — whose difficulties have become the stuff of headlines — is that they are exhausted and at their wits' end.

Armies wear out rapidly under the unique stress of combat, and for a decade now U.S. ground troops have been rotated through three, four and five combat tours lasting up to a year in each case (15 months at the height of Iraq fighting).

This is not an excuse for the soldier who ran amok in southern Afghanistan on Sunday, killing 16 civilians, many of them children; nor for the recent burnings of the Qur'an or any of the other almost incomprehensible incidents we regularly hear about in Afghanistan or Iraq. 

But at the same time we need to appreciate the high level of mental illness, substance abuse and severe depression that is ravaging American ranks and making such incidents a constant risk.

No other NATO units comes close to the length of time U.S. troops have spent in these wars, nor has anyone else taken anything like the grinding number of casualties. 

The Canadian army was seriously tired when it withdrew from Afghanistan combat last year — yet our ranks could only imagine the far greater strain on U.S. soldiers.

Since 2001, over 6,200 U.S. soldiers have been killed in Iraq and Afghanistan; over 47,000 have been wounded.

What's more, almost three-quarters of these casualties have been borne by ground troops and, of these, over 30 per cent suffered serious brain and spine injuries.

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The long ride home, in December 2011. (Reuters)

'Trauma in the mind'

To me it has always seemed shocking that Washington's current political class, relatively few of whom served in the military, have been so careless in allowing their military units to wear down like this. Perhaps because the generals constant "can do" mantra tends to blot out the reality of exhaustion.

Now, however, a few senior voices, such as former national security adviser Brent Scowcroft and former defence secretary Robert Gates, are speaking out about the human cost of these campaigns and of sending large land armies abroad.

And this week, a former commandant of the prestigious U.S. Army War College, retired Gen. Robert Scales, wrote in the Washington Post that if someone wants to place blame for the Kandahar shooting "it should be on a succession of national leaders who fail to recognize that combat units, particularly infantry, just wear out."

A Vietnam veteran, Scales believes today's vets suffer even more than his generation because "close fighting in Iraq and Afghanistan was more pervasive and lasting, thus more likely to cause personal trauma in the mind."

As a reporter, who has covered wars, including Afghanistan, I have a sense of the trauma he is speaking of. 

I was always struck by the constant level of tension that surrounded our soldiers whenever they left a main base.

It seemed quite different from conventional wars where there were always areas away from the front lines where a soldier could relax a bit. 

In Iraq, troops called it 360/365, for the stress came from not knowing from where or when the next firefight would come  — 360 degrees, 365 days a year.

A social time bomb

President Barack Obama clearly wants to bring the last big contingent of his Afghan troops home as soon as decently possible — by 2014 at the latest.

But the statistics suggest that this will inevitably transfer the problems of an emotionally drained army to the home front, where many have already taken the brunt of the economic crisis.

Up to 30 per cent of returning soldiers develop serious mental health problems within three to four months of coming home. Up to 25 per cent suffer from post-traumatic stress disorder, which many psychologists feel is an underestimation. 

In recent years it has not been uncommon to have more than 300 suicides annually by U.S. veterans, with failed attempts running at over 1,500 a year.

The Pentagon has also been fighting an epidemic of sexual assault among troops. It reports an astonishing 19,000 military men and women were sexually assaulted by fellow soldiers in 2010 alone.

Meanwhile, rates of domestic violence surged by over 30 per cent among military families between 2006 and 2011, along with drug and alcohol abuse, rates that Gen. Peter Chiarelli says has grown out of a length of combat "our nation has never experienced before."

And if that weren't enough, the growing fear of violence on U.S. bases has been heightened in recent years by a steady infiltration of street gang members into the military. 

Every major gang in America is represented on domestic and foreign bases, according to the FBI.  Some members are said to have joined up to try to escape gang life. But at least some seem intent on picking up military skills and more deadly weapons.

So I fear this latest crazed act of a deeply disturbed U.S. soldier is not going to be the last of its kind.

Too many tired and joyless men, who have been soldiering since they were teens, are still fighting for a cause that few understand, alongside fickle allies and among a population that partially despises them and their culture.

An exhausted army is prone to disasters, and this one has surely been pushed to the limit.