For years, sentinels guarding the Tomb of the Unknowns at Arlington National Cemetery voluntarily had their lives defined by four constant and silent witnesses: the Unknown of World War I, the Unknown of World War II, the Unknown of the Korean War, and the Unknown of the Vietnam War. Until 1998. That’s when the Unknown of the Vietnam War was identified as First Lieutenant Michael Joseph Blassie. The tombs—the first of which was erected in 1921represent the American soldiers who died in conflict and were never identified. Blassie was originally tallied as one more unidentified service member lost to the war, either missing or killed in action. In the longer course of history, however, he came to occupy a place at the nexus of old and new in how the United States cares for its dead.

Major James Connally spotted Blassie’s plane as it went down outside An Lộc on the morning of May 11, 1972—28 days into the battle for which the city gave its name. “The aircraft flew a short distance on its own and then slowly rolled over, exploding on impact in enemy-held territory,” Connally later recalled in a letter to the Blassie family. Though Connally knew the site of the crash, recovery took nearly six months from when anti-aircraft fire clawed the A-37 Dragonfly from the sky to when a South Vietnamese Army patrol eventually found some remains, an ID card, a beacon radio, and other small fragments of an identity. Though the materials found were enough to initially mark the remains as Blassie’s, a flawed bone fragment-based forensics process later overruled this verdict by miscalculating the supposed height of the individual to which the fragments belonged. It would take another 26 years before Blassie completed his odyssey from An Lộc, to the Tomb of the Unknown of the Vietnam War, to the Jefferson Barracks Memorial Cemetery near his childhood home of St. Louis, Missouri.

A rise in both care and capability borne out over centuries of warfare has caused the number of unidentified to gradually dwindle. Only three individuals who took part in Operation Iraqi Freedom from 2003 to 2010 have yet to be accounted for, according to the Defense POW/MIA Accounting Agency, which is responsible for the recovery of missing personnel. It’s a considerable drop from the thousands of American service personnel unaccounted for from past wars. The reasons for the decline are varied, but include both the changing character of war as well as advances in technology like DNA testing, which have made unknowns largely a phenomenon of the past. “The generation that the tomb was built for, and the unknown soldier honored and buried here … that generation of people has come and gone,” said Sergeant of the Guard Paul K. Basso.

After World War I, calls for the repatriation of bodies, which lay on the fields of Europe, flooded politicians’ offices in Washington, D.C. To many who had lost loved ones on hotly contested grounds or through the ubiquitous flash of an artillery explosion, even the full brunt of the government’s efforts to send home their loved ones often wasn’t enough. For these families, whose mourning was rendered incomplete, the nation needed something universal that could represent that final missing part of the process—a body. Though the U.S. had previously established tombs of the unknown for some 2,000 Civil War dead gathered from the fields of Bull Run and near Rappahannock, this new creation would represent those who could never be brought back, much less identified. The U.S. turned to its allies, whose own losses dwarfed America’s in the war, to seek inspiration for this memorial.

According to Bill Niven, a professor of contemporary German history at Nottingham Trent University, England, the effect of World War I on how countries memorialize conflict was a cultural turning point—one most neatly embodied in the sharp contrast between France’s modest Tomb of the Unknown and the imposing Arc de Triomphe. Constructed in the early 1800s, the arch memorializes the “glory of [Napoleon’s] Grand Armée,” while the tomb that rests in its shadow, and built more than a century later, has a subdued visage. The arch reflects the aggrandizement of war through extravagant uniforms, neat battle lines, and the ever-present murmur of honor and fidelity, but World War I had trod such formal conceptions through the muddy trenches of France and the Eastern Front. And it was that more desolate aspect of war that the tomb personifies. Here were average citizens—rather than professional soldiers—charging, fighting, and dying seemingly at random and on an industrialized scale few at home could fathom, much less fully comprehend. War itself had been radically altered, and so too had the mourning of those lost to it.

“These guys will all tell you, every single one of them, they will tell you they don’t do it for the badge,” Captain Jean J. Gwon said of the Honor Guard Badge, one of the rarest awarded in the U.S. Army. “It’s always for the unknown.” As she spoke, Gwon, the commander of the Tomb Guards, motioned to an 18-year-old soldier walking through the door to the subterranean guard quarters before his shift. The quarters, where three “reliefs” of seven sentinels spend 24 hours on duty before two days’ rest, lie beneath the risers where visitors to Arlington gather from 8 a.m. to 5 p.m. daily. Visitors watch the soldier on duty take 21 measured steps past the tomb and back again, over and over during a shift; it’s a tacit nod to the military’s highest honor: the 21-gun salute.

Near the hour mark, the crowds swell, standing on tiptoes to catch the intricate changing-of-the-guard ceremony, a moment when the unit’s strict standards are performed as each new guard takes the post. “If there was no guard, people would not come here,” Basso lamented. “People would not know what the tomb is. There would not be Facebook posts; there would not be memes; there would not be YouTube videos.” James Mayo, a professor of urban planning at the University of Kansas, explained the attraction in a 1988 article on war memorials: “Rituals transform a landscape and the memory associated with it … Through rituals, people can focus on war memory, and their performances temporarily renew the importance of these memorials in the landscape.”

And so, in January 1998, when CBS Evening News reported that Blassie was indeed the Vietnam Unknown, Americans had to collectively reconsider the role of memorials and rituals dedicated to the unknown. In an interview, Blassie’s sister, Patricia, called for a DNA test of her brother’s remains to confirm the CBS report. The military obliged. On May 14 of that year, the fourth unknown was exhumed from his resting place at the tomb for testing, and just 47 days later, Assistant Secretary of Defense Kenneth Bacon notified the Pentagon press corps that the link between the Vietnam Unknown and Blassie was absolute. It was that combination of shoddy forensic science and the lack of other suitable candidates that likely marked Blassie unidentified in the first place.

Where the airman once rested now lies a cenotaph in Arlington. The empty tomb’s cover, initially designed for the Unknown of the Vietnam War, instead reads: “Honoring and Keeping Faith with America’s Missing Servicemen, 1958-1975.” Because science virtually assures that any unidentified body could one day become identified—and thus known—then-Defense Secretary William Cohen decided that Blassie’s former tomb should remain vacant. No remains would be interred in the tomb, said Undersecretary of Defense Rudy de Leon, unless “it can be assured in perpetuity that the remains of the American serviceman would be forever unidentifiable.” And so, the tomb was sealed from gaining another member. But that hasn’t stopped its continued evolution in the minds of millions of Americans who visit it annually.

“Today, the tomb and the unknown soldiers continue to serve their original purpose, and that’s important,” said Basso, “but they also serve a whole new purpose for many Americans and the world.” War losses may now be identifiable, but they are no less poignant or profoundly grieved. After all, when visitors make the trek up to the tomb, take in the landscape, align their various screens to best capture the grandeur of the space, and hit send on their social-media accounts, those crypts—including the fourth—lie in full view. And maybe that’s enough.