The Field of Empty Chairs at the Oklahoma City National Memorial & Museum in Oklahoma City on June 6. (Nick Oxford for The Post)

Every summer my husband and I drive thousands of miles across the country and back. The reason is the dog, Augie, who can’t fly. But we are the beneficiaries.

Never more than this summer, and not just because we had another chance to savor the landscape — most striking in this year’s route, the savage beauty of South Dakota’s Badlands. But our travels, more by happenstance than careful planning, also offered sobering lessons in the unfinished business of our country, business both centuries old and still being sorted.

One striking thing was how our stops stitched together events so searing you remember where you were with pinpoint precision: In Memphis, the Lorraine Motel; at 9, I was sitting on the plaid couch in the den, watching as the news that the Rev. Martin Luther King Jr. had been shot flashed across the television screen and racing to the kitchen, where my mother was tidying up after dinner, to report. In Oklahoma City, the bombing memorial and museum; at 36, I was on bed rest with our first child when my husband called to tell me to turn on CNN because something terrible had happened. In Shanksville, Pa., the Flight 93 memorial; at 43, I was at a meeting at my children’s school when someone burst into the room to announce that a plane had hit the World Trade Center.

In those moments, the world changed and the implications still reverberate: our continuing struggle for racial equality; the related and persisting threat of white supremacy and domestic terrorism; the still-looming, indeed newly revived, danger of Islamist extremist terrorism. But there were also monuments to our collective capacity to achieve wonders, as well as the manifold ways in which we have fallen short of our national ideals.

Our first stop was at the Biltmore in Asheville, N.C., a gaudy monstrosity that serves as a reminder that inequality is a persistent American phenomenon. At 178,926 square feet — a mere 135,280 of which is living area — the Biltmore was erected in the late 1880s by George Washington Vanderbilt II and remains the largest privately owned house in the country. The absurd pile stands today less as a shrine to the Gilded Age than a measure of how much work on economic inequality remains undone in our own time.

Next came Memphis. So many years later, it was still chilling to see that fateful balcony, that vintage motel sign frozen in time. And to think, more than a half-century after his assassination, how much of King’s vision remains unfulfilled, how much the scars of slavery and Jim Crow and racism remain unhealed. What would King make of where we are now? His home state has a Black senator — and a restrictive new law that makes it a crime to give water to people standing in line to vote.

On to Oklahoma City, where the museum’s jarring opening exhibit features a tape recording of the Oklahoma Water Resources Board meeting across the street. For two excruciating minutes, you hear bureaucratic business as usual; you brace for the explosion that you know will come, and cannot help but cringe when it finally does. Outside, empty chairs — 19 child-sized — represent the 168 killed.

The bombing of the Alfred P. Murrah Federal Building in April 1995 was our national introduction to the danger of domestic terrorism but not, sadly, our last. It is easy to trace the road from Timothy McVeigh — with his anti-government delusions, fixation on Second Amendment rights and sympathy with white supremacists — to the Jan. 6 insurrectionists.

Our return trip, on a more northerly route, provided a fitting bookend, starting with Little Bighorn in Montana, the site of the battle between George Armstrong Custer’s 7th Cavalry and the Lakota Sioux and Northern Cheyenne, led by Sitting Bull and Crazy Horse, over two bloody days in June 1876.

If slavery is our nation’s original sin, so, too, is its treatment of Native Americans — slaughtering them, taking their land, herding them onto reservations. We remember the battle as Custer’s Last Stand — Custer and nearly 300 of his soldiers were killed — but it might be better understood to represent the tribes’ fierce resistance to outsiders set on destroying their way of life.

Neighboring South Dakota offered two unrelated monuments to American persistence and ingenuity. We saw Mount Rushmore by night, the faces of the presidents carved into the granite over 14 years. Adjoining the nearby Badlands, with cattle grazing beyond the fence, a decommissioned Minuteman missile silo. It is one of 150 buried across the desolate prairie, part relic, part reminder of a still-dangerous world.

Our final stop, at the Flight 93 memorial, was perhaps the most unsettling, given not only the proximity of the 20th anniversary of 9/11 but also the terrible events unfolding in Afghanistan even as we read the names of the 40 passengers and crew members who died there. What would the brave souls who took down the plane in a Pennsylvania field and likely saved the U.S. Capitol have to say about the U.S. mission in Afghanistan and its gruesome end? Would they be relieved, knowing that another such attack on U.S. soil had not succeeded in the ensuing years — or furious to learn that the very group that had shielded their attackers was back in power?

We drove the rest of the way home in silence, feeling both at once, Augie resting his head in the space between us.