The latest untoward event in the U.S. military expedition in Afghanistan is horrifying any way you look at it. A U.S. Army staff sergeant walked off his base Sunday and, going door-to-door in nearby villages, shot to death at least 16 civilians, including nine children, subsequently setting some of the bodies afire. We Americans can be confident that whatever sort of derangement accounts for this act does not reflect official policy or orders, and that, as our leaders like to reassure others, the action is not representative of the large majority of American military personnel serving in Afghanistan. But what we can be confident of is not necessarily what matters most.

The history of the U.S. military involvement in Afghanistan and of similar expeditions elsewhere—including the Iraq War, punctuated by the abuses at Abu Ghraib—demonstrates two realities about such incidents. One is that in any wars as long and large as these two, such incidents are bound to happen. Their occurrence is a function of having many thousands of men and women in uniform sent to a foreign land to perform the mission, the inability to subject all of them to constant supervision, and the statistical likelihood of aberrant behavior stemming from the weaknesses of any assemblage of people that large as well as from the extra unusual stresses of warfare. Incidents will occur despite reasonable efforts of the command to exert discipline and to prevent them from occurring. The inevitability of their occurrence is part of why we have a military justice system.

The other reality is that many foreigners will interpret the incidents differently from the way we do. They will interpret them as more willful, and more representative of Americans or of what the United States is trying to do, than is the case. The United States suffers from its own power by being the target of assumptions that it always can do whatever it wants to do and can prevent whatever it does not want to happen. No amount of explanations, apologies, or reassurances from U.S. leaders will dispel such perceptions.

The latest incident is one of several that, along with the unpopularity of some of the NATO forces' tactics and the sheer length of those forces' presence in Afghanistan, has made the Western military presence progressively less welcome in that country. The negative Afghan sentiments involved have made it more difficult and dangerous for NATO personnel to do their jobs, as highlighted by the growing number of murders of those personnel by Afghans they are supposed to support and advise. It remains to be seen if reaction to Sunday's shooting spree will be anything like the Afghan response last month to the burning of Korans. If it turns out that the accidental burning of a religious book elicits more anger than the massacre of more than a dozen innocent villagers, it will be one more demonstration that we and the Afghans operate on different wavelengths.

The only appropriate policy response to these developments is to press ahead with military disengagement from Afghanistan. The Western mission already has become very hard to perform, and there are bound to be more incidents that will make it even harder. And yet, some of the same tired arguments for doing otherwise continued to be voiced. Senator John McCain talks of how “if Afghanistan dissolves into a situation where the Taliban were able to take over a chaotic situation, it could easily return to an al-Qaeda base for attacks on the United States of America.” This ignores how the Afghan Taliban, which is not an international terrorist group and cares only about the political and social order inside Afghanistan, has strong reasons not to make the country an al-Qaeda base and suffer again the same kind of fate it did in late 2001.

Then there is Senator Lindsey Graham stating “we can win this thing” and saying that leaving Afghanistan would signal to Iran that the United States was not committed to the region. The idea of hurting the credibility of commitments is an even hoarier notion, one that was very much in evidence in continuing to fight the Vietnam War. It is no more valid now than it was then, and is not the way we would assess the commitments of other states. As for what being in Afghanistan does to Iran, that's another lesson we should have learned from the Iraq War—which was one of the biggest boosts to Iranian influence in the region that Tehran has enjoyed.