Lieutenant General Stanley Mc- Chrystal, President Obama’s choice to be the next military commander in Afghanistan, told senators earlier this month that “the measure of effectiveness will not be the number of enemy killed. It will be the number of Afghans shielded from violence.” If General McChrystal can achieve this, he will have a far better chance of turning around a war America has not been winning - but must.

It isn’t just Taliban violence that Afghans need shielding from. Errant American fire has taken an unacceptably high toll, especially from the airstrikes that American commanders came to rely on because they lacked sufficient ground troops. One particularly deadly episode last month killed dozens of civilians (the Pentagon says 20 to 30; the Afghan government says 140).

The New York Times recently reported on the initial conclusions of a Pentagon investigation, citing significant errors by military personnel that contributed to the high civilian death toll. These included ignoring a rule against bombing high-density residential areas in the absence of imminent threat and failing to reconfirm a target after a bombing delay.

Such mistakes are costly, not just in civilian lives but in broader support for the presence of American troops and the military campaign against the Taliban.

Afghanistan’s people have little enthusiasm for a war in which foreign troops and Taliban fanatics shoot at each other with seeming indifference to the civilians caught in the cross-fire. Last year, some 2,000 Afghan civilians were killed, according to the United Nations and private aid agencies.

Reducing that toll will require tighter and more strictly enforced rules of engagement. That applies not just to airstrikes but to the search and detention operations that General McChrystal wants to expand this year with the help of 21,000 additional troops that President Obama ordered sent to Afghanistan. Ground operations are less likely to go astray than airstrikes. But as happened far too many times in Iraq, they can sweep up innocent civilians and turn local people against the American presence.

Protecting Afghan civilians, and expanding the secure space in which they can safely go about their lives and livelihoods must now become the central purpose of American military operations in Afghanistan. And Washington must step up the pace and quality of training so that expanded Afghan military and police forces can take over that mission as soon as possible.

The right words are being said in Washington. Now they must be applied in Afghanistan.