A U.S. soldier and the Afghanistan soldier who killed him KABUL, Afghanistan - In many ways, the two young soldiers were not so different from each other. Each was tough-minded and physically powerful. Each worked hard to win a place in an elite military unit, and spoke with pride of serving his country. They were 25 years old, these two: one newly married, the other planning a wedding this year. Their upbringings were as disparate as their homelands were distant, but religious faith was entwined with the family lives of both. Their lives ended, violently and nearly simultaneously, one evening late last month at a remote outpost in southern Afghanistan - one dead at the other's hands. An Afghan special forces sergeant named Zakirullah has been identified by his commanders as the man who shot and killed U.S. Army Staff Sgt. Andrew Trevor Britton-Mihalo, a Green Beret from Simi Valley, before being gunned down himself. "We had thought he might die while he was serving in the army," said an uncle of Zakirullah, who, like many Afghans, used one name. But we never thought it would happen like this. At the fatal intersection of these two young lives, American and Afghan, lies the heartbreaks of the Western presence in this country, and the many ways in which two ostensible allies have both buoyed and failed each other. As the U.S. military embarks on the task of extracting itself from America's longest war, the phenomenon of members of the Afghan security forces turning their guns on Western troops is becoming, in the eyes of some commanders, a strategic threat. At least 21 NATO troops have died in these assaults this year, accounting for a stunning 14% of troop deaths, with the latest shooting coming Friday in northeastern Afghanistan, the victim an American. The total number of such attacks is unknown; they generally go unreported publicly by the military if they result in injuries only. Although all have a corrosive effect on field morale and trust, the April 25 confrontation that killed Britton-Mihalo was cause for particular alarm: For the first time, the killer was a member of Afghanistan's special forces, handpicked from the ranks of its commandos, who are themselves considered an exclusive fraternity: carefully vetted, highly trained, closely watched. With the NATO force preparing to end its combat role in Afghanistan, heavily freighted hopes are riding on the U.S.-mentored Afghan special forces. They are being groomed to take the lead in nighttime raids, which have proved to be perhaps the single most effective tool in killing and capturing leaders of the Taliban and other insurgent groups. More than three dozen teams of Afghan special forces are spread across the country, partnered with U.S. counterparts in "village stability" operations meant to win the support of residents in isolated hamlets menaced by the Taliban. In outposts like the makeshift base in Kandahar province where Zakirullah and Britton-Mihalo were deployed, the punishingly primitive, close-quarters conditions can foster strong bonds - or allow small irritants to fester. "They work together, they patrol together, they are together all day of every day," said Col. Bismullah Waziri, executive officer of the Afghan commando brigade. We are all aware that there are cultural issues. Sure, they are different from us. And we are different from them. Britton-Mihalo's military roots ran deep. His dad was a Marine; Andrew was born in Costa Rica while his father was serving there. In December, he married Jesse Lamorte, an Army combat photographer who had served in Afghanistan with a special operations unit. He had proposed to her stateside, at a Special Forces winter ball. Two of his half brothers recently enlisted, and Britton-Mihalo had signed up for a third tour in Afghanistan. From the beginning, his career aim was soldiering. He signed on with the Army in 2005, right after graduating from Royal High School in Simi Valley. Nobody who knew him was surprised when he made it into the elite Green Berets in 2008. "He was something special when it came to dedication and endurance," said Paul Mole, one of his wrestling coaches in Simi Valley, where a 5-year-old Andrew had moved after his parents split up and his mother remarried.