KABUL, Afghanistan -- It can be a split-second decision, or one that plays out over long and agonizing hours: to kill or not to kill.

"Rules of engagement" is the dry, legalistic term for the visceral battlefield calculus of when and whether to use deadly force to counter threat, real or perceived. Across Afghanistan, these rules serve as the marching orders that govern Western troops' daily encounters with Taliban fighters -- and color dealings with Afghan civilians.

U.S. Army Gen. David Petraeus, who on Sunday formally took command of Western forces in Afghanistan, must decide in the coming weeks or months whether to recalibrate the stringent rules of engagement laid down last summer by his predecessor, Gen. Stanley McChrystal, who recently resigned over remarks that laid bare a dysfunctional civilian-military relationship.

Of Gen. Petraeus' early command decisions, this will be among the most closely watched, not only by ordinary Afghans, but also by his own troops in the field.

Assuming command on Sunday, Gen. Petraeus told his troops that while civilian safety remains a critical consideration, "as you and our Afghan partners on the ground get into tough situations, we must employ all assets to ensure your safety."

It was a remark intended to reassure those in the field that the safeguarding of Afghans was not to come at the expense of military lives.

When Gen. McChrystal took over as commander in June 2009, foreign forces in Afghanistan were the accidental cause of nearly as many civilian deaths as were the insurgents, who often deliberately put noncombatants in harm's way.

Gen. McChrystal set out to change that, and was credited with bringing about a substantial drop in the proportion of civilian casualties suffered at the hands of NATO's International Security Assistance Force and its Afghan allies.

Under the procedures put in place last summer, commanders could not fire on buildings or other sites where they had reason to think Afghan civilians might be present unless their own forces were in imminent danger of being overrun. And even then, they were told to break off engagements and withdraw rather than risk harming noncombatants.

Civilian casualties have been a particularly sensitive issue between the Western coalition and Afghan President Hamid Karzai.

Sunday, though, brought a fresh reminder that for civilians, the insurgency usually poses the greatest threat to life and limb.

In southern Afghanistan's restive Helmand province, Afghan security forces were summoned to deal with an urgent threat: a bomb that had been planted on a donkey -- the ubiquitous beast of burden in Afghanistan -- in a bazaar in the district center of Musa Qala.

As the explosive device was being disabled, another bomb affixed to a motorcycle went off elsewhere in the bazaar, provincial spokesman Daoud Ahmadi said. Four people, some of them said to be young children, were killed and about six others were wounded.

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