Azra Bashir spoke by phone to her son Usman, holed up in a freezing border outpost on the dangerous Afghanistan-Pakistan border, only hours before NATO airstrikes killed him and 23 other Pakistani soldiers. The 23 year-old captain told his mother not to worry about him, and reminded her to watch her health.

As Pakistan and the United States argue about the sequence of events that led to the attacks, Bashir is struggling with the pain of losing a child. Her anger, and that of other relatives mourning loved ones killed by a nominal ally, helps explain the uncompromising stance Islamabad has struck toward Washington since the incident.

"I want to tell our soldiers that they should avenge the killing of Usman and other soldiers like him," Bashir said in an interview in her home in Punjab province. As she spoke, she kissed a framed photo of her son, who also left behind a wife and 2-month-old daughter.

Bashir's call for revenge has been echoed in daily protests held in Pakistan's major cities, many of them organized by Islamist and right-wing parties who have long said that America and NATO -- not the Taliban -- are the prime enemies of Pakistan.

The border incident has greatly strengthened that narrative, reducing the political space for those who argue that cooperation with Washington is in the country's interest. The army, which has received billions of dollars in U.S. aid since 2001 in exchange for its cooperation, however limited, against militants, has fueled the hard line by accusing NATO of a "deliberate act of aggression."

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The 24 deaths by apparent American friendly fire come on top of the 3,000 Pakistani security force members who have been killed over the last 10 years fighting insurgents, mainly in the northwest close to the Afghan border.

Many in the country, including leading politicians, say the war has been foisted upon them by America. They say the violence would end if Islamabad severed its ties with Washington.

"How long will we sacrifice our youths, our soldiers for others?" said Capt. Usman Bashir's father, Bashir Ahmed. "This is not our war. This is their war."

American and NATO officials have expressed sympathy over the deaths, saying the incident was a mistake and is being investigated. The border area is infested with militants, whom NATO has long complained receive safe haven on the Pakistan side to launch attacks in Afghanistan.

"What kind of mistake is this that kills innocent poor people?" asked Asfandyar Khan, who lost his 22-year-old son in the attacks. "We don't want your investigation and inquires. I want justice. I want real action against those responsible."

He fought back tears as he spoke, sitting feet away from the freshly dug grave holding his son, Najibullah Khan.

The grave in the family's village of Kabuli Kili in northwest Khyber Pakhtunkhwa was covered in wreaths of flowers donated by military officials. A green and white Pakistani flag flew from a bamboo pole nearby.

"You are calling us friends," Khan said of the United States. "Is this the way you people treat friends? If this is friendship, we have had enough, and don't want such friendship."

Pakistan has retaliated for the incident by closing its Afghan border crossings to NATO supplies, by demanding the U.S. vacate an air base used by American drones attacking militants along the frontier, and by boycotting an international conference in Germany aimed at stabilizing Afghanistan.

Washington is keen to repair damage done to the relationship. It wants to get the supplies moving again, and also sees Islamabad's links with Afghan insurgent leaders on its soil as a key asset in negotiating a peace deal in Afghanistan which will allow the U.S. to withdraw its combat troops by the scheduled 2014 deadline.

Pakistan relies heavily on American aid, and it too wants to avoid a rupture in ties.

The U.S. and Pakistan have long had a troubled relationship, thanks to Pakistan's reluctance to target Afghan Taliban fighters and their allies using Pakistani territory to attack American troops in Afghanistan. Islamabad is believed to see those insurgents as useful proxies in Afghanistan, once the U.S. withdraws.

The NATO attack was the latest in a series of crises to beset the relationship this year.

In January, an American CIA contractor shot two Pakistani men who he said tried to rob him, sparking outrage. The May 2 unilateral raid that killed Osama bin Laden was also portrayed as a gross violation of Pakistan's sovereignty, largely drowning out questions over how the al-Qaida chief was living undetected in an army town for five years.

Most experts believe the two countries will patch things up this time, and that the border closure will be temporary, chiefly because Washington and Islamabad still need each other. But the Pakistani reaction since the strikes has betrayed the lack of trust at the heart of the relationship, and bodes ill for meaningful Western cooperation with Pakistan over ending the Afghan war.

"The time has come for 180 million Pakistanis to choose between a life of respect or ignominy," said Shahbaz Sharif, the head of the ruling party in Punjab after visiting Bashir's family this week.

"American foreign aid is drenched in the blood of martyrs and we will have to give it up and get back up on our own feet," he was quoted by local media reports as saying.