In Afghanistan, the killings are called "green on blue" — that's when an Afghan soldier or police officer turns his gun on a NATO ally.
There was a wave of such violence just last month after U.S. soldiers accidentally burned Korans. Over the next week, six Americans were killed, apparently at the hands of Afghans working with the U.S.
The top U.S. and NATO commanders in Afghanistan think they have some answers to this recurring problem, and it's up to U.S. soldiers like Capt. Joe Fritze to see if they work.
Fritze heads up training patrols with Afghan police, and they work the neighborhoods of south Kabul like cops on the beat.
Fritze does what he can to put Afghan residents at ease, speaking to them in his basic Dari — or at least trying.
"I just introduce myself, ask them about their families," he says. "I ask for tea. I always say 'thank you' — 'tashakkur.'"
But the captain himself is not quite at ease on these walks.
Fritze says that the very people he's training, the Afghan National Police, or ANP, could pose the greatest danger to his soldiers.
"You always have to keep the thought in the back of your mind that something bad could happen — not coming from the populace but coming from the ANP," he says. "And you have to protect yourself."
A Long-Standing Problem
The problem goes back years. More than 70 members of the NATO coalition have been killed by men in Afghan police or army uniforms in the last five years.
NATO downplayed the problem until a shooting in January where four French troops were killed by a rogue gunman. France then insisted that something be done.
So NATO asked the top commander in Afghanistan, U.S. Gen. John Allen, to come up with ways to prevent such attacks. The shootings have been carried out by members of the Afghan security forces, as well as Taliban members who infiltrated the forces or disguised themselves with uniforms.
Allen put together a report that included some new ideas as well as measures that were already on the books.
The first step on the Afghan side is to do a better job of screening applicants for the army and the police.
There's already an eight-step vetting process. It requires character references from tribal elders, community guarantees and biometric screening. But the Afghans weren't following the process, according to the U.S. military.
"The measures that are going into place were not fully embraced on the Afghan side, they were not fully implemented. I believe they are now," Allen says.
The next challenge comes once an Afghan gets in the security forces. It involves watching for signs that someone is being radicalized by religious extremists, which can happen when some Afghan soldiers or police go on leave.
Until now, they've faced no questions when they return. Allen wants that to change.
Searching For Radicals
There's a more covert way they're addressing the radicalization problem. Three hundred Afghan counterintelligence operatives are being deployed inside the Afghan army.
"We will have better surveillance and observation for any abnormal activity," says Afghan Defense Minister Abdul Rahim Wardak.
American and NATO troops will also have to watch out for unusual activity. Allen says he's reminding troops to keep their guard up around their Afghan counterparts.
This poses something of a dilemma: Allen needs to train the Afghan military so it can take the lead security role over the next couple years. That means building trust among close teams of Afghans and American and NATO troops.
But to guard against killings carried out by Afghan forces, the imperative now is: Don't trust too much.
That can actually be a challenge for soldiers like Capt. Fritze, the American training commander at Camp Julien, near Kabul.
"I would trust these guys with my life," he says of his Afghan interpreters and their ability to interpret not just words but body language and culture.
Fritze thinks of these men as his personal counterintelligence system.
Gul Agha Shirazi, one of the interpreters, sees his responsibility the same way.
"Whenever we see any sort of suspicious thing we have to report it to the captain immediately ... for our safety and their safety," he says.
The captain says he is lot more wary of the Afghan troops he's training.
"You work to build that trust," he says. "No matter what you do to build that trust ... if one of the soldiers wants to pull a gun on us, he'll pull a gun on us. And it's our responsibility to be ready for that."
In an effort to be prepared, every time the captain goes out on a training patrol with the police, he takes twice as many coalition soldiers as Afghans.