The increasing frequency of attacks on U.S. and NATO troops by members of the Afghan security forces they are supposed to be helping has reached the point where American commanders are rightly concerned that the mistrust engendered by such incidents threatens to interfere with the completion of their mission. That has prompted a shift in policy that may or may not work, but in any case it underscores the reality that the sooner our troops get out of Afghanistan the better off we'll be.

So far this year 39 coalition soldiers have been killed in attacks by Afghan men wearing army or police uniforms. That is more than all of last year and more than the entire three-year period between 2007 and 2010. The upsurge in so-called green-on-blue violence has prompted NATO issue new guidelines intended to help coalition troops better protect themselves from the Afghan forces they are partnered with, but they also risk putting up new barriers between NATO and Afghan soldiers that could produce the opposite effect.

The new rules require U.S. and NATO soldiers to carry their weapons loaded at all times so they can react more quickly if an attack occurs. Analysis of previous incidents suggests that even a few seconds can mean the difference between life and death. In addition, from now on one or two soldiers in every unit will be designated as so-called "guardian angels." Their main task will be to watch the Afghans their units interact with during every operation or meeting.

But some commanders fear that even these changes may not solve the problem. That's because U.S. officials remain basically at a loss as to why these attacks are occurring. Only a handful of the coalition deaths this year can be definitely attributed to Taliban infiltrators. The vast majority of cases appear to have been the result of violent personal and cultural clashes between new Afghan recruits and their coalition trainers.

Many of those attacks are carried out by Afghan soldiers from remote rural areas who resent the presence of foreigners in their country even more than they dislike the Taliban. They bristle at being bossed around by Western drill sergeants using foul language that they interpret as disrespectful of their customs and religion, and they react to such perceived insults according to their traditional code of honor.

Yet even that explanation doesn't fully capture the complexity of the problem, because even as incidents of green-on-blue violence have swelled, the number Afghan security personnel killing each other has also risen; there have been at least four such killings so far this year. Some Afghan leaders suggest their society simply has become more violent for everyone as a result of 30 years of nearly continuous warfare.

Whatever the reason for the sudden increase in Afghan army and police attacks on coalition forces, the development poses serious problems for NATO's plans to transfer responsibility for the country's security to the Afghans when its forces leave in 2014. Commanders worry that morale among coalition troops could drop even further as our soldiers become less willing to risk their lives for people who they don't trust and who don't want them there in the first place.

No one expects that situation to improve any time soon, and it may get worse. This is a time for the U.S. to rethink exactly why we need to keep such a large ground force in Afghanistan for another two years. Unless there's a realistic prospect of accomplishing substantially more in the next 24 months than we've been able to accomplish over the last 10 years, perhaps we've already done all that is realistically possible there and it's time to consider speeding up the timetable for bringing all the troops home.