The honor guard prepares to fold the flag over the casket during the funeral of fallen US Army Cpt. Humayun Saqib Muazzam Khan at Arlington National Cemetery in Arlington, Virginia on June 15, 2004.  (Photo by Gerald Martineau/The Washington Post)

More than any other dust-up or misstep of the past few weeks, the debate sparked by Khizr Khan’s charge against Donald Trump — “You have sacrificed nothing, and no one”— has shaken the Trump campaign and many Republicans.

The reasons are obvious. Trump’s decision to go on the offensive, publicly criticizing Khan and his wife Ghazala, cut against the grain of contemporary American politics where war dead and their survivors, especially Gold Star families, are off limits.

But what risks being lost amid the reality-show atmosphere of this presidential campaign is a more fundamental polarization: the “widening gap” in military-civilian relations in the United States. The changes in the military introduced at the end of the Vietnam War, including the transition to an all-volunteer armed force, have helped to foster a sense of separateness, even alienation, further heightened in the wake of the 9/11 attacks.

Because of these changes, the U.S. military, and especially its fallen service members, have come to occupy increasingly sacred ground, set apart from civilian life and civilian death.

However, this wasn’t always the case.

Just ask the Vietnam veterans who stream through the capital on gleaming Harleys as part of Rolling Thunder’s commemorative ride on Memorial Day. For many of them, their homecoming from Southeast Asia was met with hostility. Moreover, they, alongside the families of prisoners of war and those missing in action, had to push the U.S. government to recover and repatriate fellow service members’ remains. Their slogan was and still is “Bring them home or send us back!”

Or take the Tomb of the Unknowns in Arlington National Cemetery. Now there are railings barring entrance to the plaza at the Memorial Amphitheater, and signs to command silence and respect, and a 72-metric-ton marble sarcophagus covering the World War I crypt.

But following its dedication in 1921, the memorial was much more open. Visitors could approach it and physically touch it. Some would even picnic beside it. The intent was to encourage a sense of connection between the living and the dead.

Today, visiting the Tomb is a very different experience, where the civilian-military gap is on full display. The public stands cordoned off from the monument, focused not to the unknowns themselves but the changing of the guard in all its synchronized perfection.

Or consider Arlington Cemetery itself, known as the nation’s “most sacred shrine.” It was established during the Civil War by placing the bodies of the Union dead on the Confederate Gen. Robert E. Lee’s estate, ensuring that Lee’s family could never return. Indigent soldiers were disinterred and reburied there, and unknowns collected from battlefields to populate its grounds. The general’s wife, Mary Lee, roiled at the thought of her garden filled with bodies and the graves “planted up to the very door without any regard to common decency.” The war dead who violated her sense of decency were the same who consecrated the Union’s new cemetery.

These shifting values seem counterintuitive. But, in fact, war dead are not fixed symbols. As anthropologist Katherine Verdery explains, dead bodies have political lives. They can evoke different things for different people in different eras. In the case of fallen service members, they can help establish (or undermine) political legitimacy.

For example, from 1991 to 2009, a blanket ban on photographic images of flag-draped coffins returning home shielded the public from war’s human toll. George W. Bush rigorously enforced the ban during the wars in Iraq and Afghanistan, while President Obama lifted it the year after he took office. To one administration, photographs of returning war dead might be a liability. To another they are essential for transparency.

As fallen service members’ sacrifice has become increasingly venerated, so has the sacrifice of their survivors. The controversy of the past week was less about Capt. Humayun Khan’s death in Iraq than Trump’s criticisms of his parents.

Why is it that families of war dead are also such powerful political symbols? The answer lies with the notion of sacrifice. Lincoln understood it well. “I pray that our Heavenly Father may assuage the anguish of your bereavement,” he wrote the mother of a missing soldier, “and leave you only the cherished memory of the loved and lost, and the solemn pride that must be yours to have laid so costly a sacrifice upon the altar of freedom.”

Last Monday, the same day Trump tweeted back against the Khans, President Obama took pains to acknowledge the sanctity of war dead, a status defined by sacrifice. At the 95th National Convention for Disabled American Veterans, in what would likely be his last major address to veterans as their president, he spoke of the country’s “sacred covenant” with the members of its armed forces.

He also made clear that this covenant extends no less to families, including and especially Gold Star families. “No one — no one — has given more for our freedom and our security,” than they have. To sacrifice is to give up something, one’s life, or, in the case of military parents, one’s child. The sacred lies in the trust that that life, that child, will be honored and remembered. Military deaths accrue debt. As Henry Ward Beecher put it a hundred and fifty years ago, in another time of national polarization, “They hover as a cloud of witnesses above this Nation.”

Much has been made of Khan’s pocket Constitution and his question about whether Trump had even read the text. But perhaps more notable was the question he posed just moments later: “Have you ever been to Arlington Cemetery?” He was questioning Trump’s understanding of the debt owed to a fallen service member — of Trump’s commitment to the “sacred covenant,” and to the sacrifice of the nation’s war dead and their families.

In the days that followed, Trump sought to push back, casting himself in the role of the victim, explaining that he had a right to respond to attacks against him. More sober voices of his party counseled him to refrain. As historian Thomas Laqueur writes in The Work of the Dead, “Attacking the dead was and is a profoundly radical step.” It too is work — “work of unmaking.”

In an era of heightened reverence for military sacrifice and sacred obligations, attacking the dead’s most visible proxies, the families of those killed in battle, is more than just impolitic. It could well catalyze the unmaking of a candidacy.

Sarah Wagner is an associate professor of anthropology at George Washington University.