COMBAT OUTPOST SANGESAR, Afghanistan >> A burst of gunfire snapped 1st Sgt. Joseph Hissong awake. Then came another, and another, all with the familiar three-round bursts of a U.S. assault rifle — and the unfamiliar sound of its rounds being fired in his direction.

The shooters were close. His first thought: “Are Taliban inside the wire?”

But it was not the Taliban. Over the next 52 minutes, as his company of paratroopers braved bullets and rocket-propelled grenades in the predawn darkness to retake one of their own guard towers in southern Afghanistan, they found themselves facing what has become a more pernicious threat: the Afghan soldiers who live and fight alongside the Americans.

The attack on Hissong’s company, on March 1 at Combat Outpost Sangesar, left two Americans dead along with two Afghan assailants, but it was not the first time that Afghan solders had attacked forces from the U.S.-led coalition, nor would it be the last of what the military calls “green-on-blue” attacks. Already this year, 22 coalition service members have been killed by men in Afghan uniform, compared with 35 for all of last year, according to coalition officials.

Yet with the coalition as a matter of policy offering only the barest of details about the attacks — the episode at Sangesar, for instance, was disclosed in a 71-word coalition statement — interviews conducted during a week at this outpost provided a rare and detailed account of the violence.

The attacks, and the personal animosity that officials believe have driven most of them, are threatening the joint-training model that is one of the remaining imperatives of the Western mission in Afghanistan. The future of that program will be a main topic at a NATO summit meeting this weekend, as U.S. and European leaders discuss whether to accelerate their drawdown plans specifically because of green-on-blue violence.

At the personal level, the Sangesar attack was a nightmarish betrayal for the units involved, and in the moments after the violence ended their commanders were already struggling to figure out how the Afghan and U.S. soldiers who share the base could possibly cooperate again.

They knew how quickly the situation could spiral downward.

Just days before, hundreds of U.S. advisers had been pulled from Afghan government offices in Kabul after two U.S. officers were killed by an Interior Ministry employee, worsening an already poisonous atmosphere during the rioting that broke out after U.S. soldiers burned Qurans.

The Afghan and U.S. officers at Sangesar, in southern Afghanistan’s opium poppy belt, decided pulling back from one another was not an option at the base. Instead, they immediately put their men to work together repairing damage from the attack.The Americans also quickly turned down an Afghan army offer to swap out the Afghan unit based at Sangesar.

Hissong’s unit — Company B of the Second Battalion, 508th Parachute Infantry Regiment, from the 82nd Airborne Division — had assumed formal command of the outpost only on the night of the attack. New to the area, the Americans reasoned they needed the local knowledge of the Afghan unit, which had been in place for some time. The base is in the Zhare district of Kandahar Province, the closest thing to home turf for the Taliban, a group founded at an Islamic seminary a few miles from the outpost.

U.S. and Afghan soldiers were back out on joint patrols within a week. Security measures imposed immediately after the attack — like posting armed guards at the U.S. mess hall — had fallen away by the end of the month.

In April, U.S. and Afghan soldiers paired up to successfully push the Taliban from a nearby village.

After watching Afghan soldiers kick down doors and clear mud-brick farm compounds, “it’s hard not to like some of those guys,” said 1st Lt. Nicholas Olivero, 24, of Fairfax, Va. “But I’d be lying if I said there was trust across the board.”

Another U.S. soldier added: “I don’t always need to have them walking in front of me now. I did for a while.”

Yet Afghan soldiers still complain of being kept at a distance by the Americans, figuratively and literally. The Americans, for instance, have put up towering concrete barriers to separate their small, plywood command center from the outpost’s Afghan encampment.

Also still in place is a rule imposed by the Afghan army after the attack requiring most of its soldiers to lock up their weapons when on base. The Afghan commanding officer keeps the keys.

One U.S. soldier nonetheless advised a visitor to take an armed escort to the Afghan side of the base, which was about 100 feet away, “just in case.”

The effort at Sangesar to move past the attack, and the difficulties in doing so, exemplifies the broader struggle that U.S. -led forces face as they seek to accelerate the training of the Afghan army and police forces to take over before NATO’s combat mission ends in 2014.

Sangesar, like hundreds of other coalition outposts scattered across Afghanistan, is split between U.S. and Afghan forces and situated on a few acres in a remote and often hostile area.

Its structures are made of little more than sandbags, heavy-duty tents, plywood huts and Hesco barriers, hulking bales of canvas wrapped in wire mesh and filled with dirt. The guard towers at Sangesar are essentially wooden frames filled out with sandbags and placed atop the base’s exterior wall of double-stacked Hescos.

Spc. Payton Jones, 19, was alone in one of the towers around 3 a.m. on March 1 when two Afghans sneaked up. They killed him with a bullet to the head.

Within minutes, Staff Sgt. Jordan Bear, 25, who was among the first soldiers on the scene, had been fatally wounded in a volley of fire from the tower. When Hissong, a 35-year-old on his third tour in Afghanistan, arrived moments later, bullets were still smacking into the ground near where Bear had fallen.

The two Afghans in the tower — a soldier and a civilian teacher — were in an easily defended position. The only approach was up a funnel-shaped stretch of open turf that gave them a clear field of fire to repulse any counterattack.

Along with assault rifles, the Afghans had a U.S. machine gun and their own rocket-propelled grenades. One RPG obliterated a sandbagged bunker between a pair of mortar pits at the center of the base, just moments after a U.S. officer had dashed out of it.

Despite the gun and RPG fire, Hissong and another soldier managed to sneak closer to the tower along a row of Hescos. But they could not take a clear shot at the tower’s narrow entrance — its only opening — without dangerously exposing themselves.

They turned to their grenade launchers but were too close to the tower for the grenades to arm once fired. Most landed with nothing more than a thud. The ones that did explode hit the tower’s exterior, inflicting little damage.

Helicopter gunships were soon overhead but could not risk firing their missiles or explosive rounds — the base’s fuel tanks were right next to the tower.

The paratroopers on the ground tried approaching the tower in an armored vehicle. But it was disabled with an RPG before it could be positioned to fire its powerful gun.

That left Hissong and his comrade. After firing 17 grenades, they were down to their last one. They tried to position themselves so they could get a clear shot into the tower — and enough distance so it would detonate.

Instead, it bounced off a wall and exploded atop a thick fuel line, sparking a fire that quickly shot toward the main fuel supply: a rubber bladder as big as a swimming pool that was now separated from the flames by only a row of Hescos.

Racing to disconnect the line from the main fuel supply, Hissong did not realize B Company had finally caught a break: Flames were also climbing the wooden stairs to the tower, filling it with smoke.

The Afghans in the tower pushed out an exterior window, jumped about two stories to the ground and ran. They made it roughly a hundred yards before being cut down by an Apache helicopter.

The fight was over. But as the Americans and Afghans at the base began to regroup, they soon learned a third conspirator, an Afghan sergeant, remained among their ranks.

At the outset of the attack, the Afghan sergeant had gone to the outpost’s entrance and shot the two guards — a fellow Afghan soldier and an American. Then he sneaked back to his bunk to wait out the fighting with the other Afghan soldiers.

His undoing: He had not killed either man at the entrance. The American was hit in the chest plate of his body armor, knocked down and badly bruised, but nothing more. The Afghan guard was shot clean through the shoulder, a serious but not life-threatening wound, and he quickly identified the third conspirator. Afghan forces detained him immediately.

The coalition and Afghan army would now have a rare opportunity to interrogate an Afghan soldier who had turned on coalition forces; most are quickly killed in ensuing firefights. Why had three men attacked U.S. soldiers they barely knew? Was it a personal grudge against Americans? Or had they turned to the Taliban?

The detainee has since presumably been asked those questions. But in a reflection of the official reticence to discuss green-on-blue attacks, his answers remain shrouded in secrecy. It is not even clear whose custody he is in.

© 2012 The New York Times Company