ARLINGTON, Virginia (CNN) -- Staff Sgt. Travis Nielsen had no idea when he joined the U.S. Army that his duty would include one of the most solemn and hallowed ceremonies in the military.



John F. Kennedy's funeral in 1963 included a riderless horse with boots facing backwards in the saddle.



During funeral processions at Virginia's Arlington National Cemetery near Washington, Nielsen walks the riderless horse, a powerful military symbol that stands among the highest honors for the fallen.



Images of the so-called caparisoned horse, often referred to as the "cap horse," remain emblazoned in the memories of millions of shocked Americans who watched President Kennedy's funeral procession shortly after his 1963 assassination.



According to Army tradition, a ceremonial horse is led by a "cap walker," like Nielsen, in a procession with boots set backward in the saddle's stirrups. In addition to high-ranking government officials such as the president, the cap horse honor is reserved for officers of the rank of colonel or above.



The tradition dates "to Roman times, or Genghis Khan," Nielsen said, "as a high honor bestowed on high-ranking fallen warriors."  Watch Nielsen lead a riderless horse at Arlington »



The ancient riderless horse ceremony didn't include backward boots, he said, but it did include an unusual meal.



"They were shrouding their horses or putting him in battle armor or escorting the fallen to their grave," Nielsen said. "When that was done, they would eat the horse, and they would have a big feast."



Today "the boots facing backward symbolize [that] the fallen won't ride again and [the rider is] looking back on his family one last time," he said.



Nielsen serves with the ceremonial Caisson Platoon of the 3rd U.S. Infantry regiment, also known as the Old Guard, based at Fort Myer, Virginia, near the cemetery. Formed in 1784, the Old Guard ranks as the oldest active duty unit in the Army.



"Memorial Day weekend is very busy around here," said Nielsen, who joins Old Guard comrades in the annual tradition at Arlington called "flags in."



"We are responsible for going out in the cemetery and placing the American flags on all the headstones."



Platoon soldiers rarely know any details about the troops or civilians they honor.



"Sometimes someone who served with the fallen or maybe went to [military] academy with them will come up to you and tell you what a great guy they were," Nielsen said.



When choosing Old Guard members, commanders "want guys who are punctual and disciplined and picky about the way they look and the way their horses look," he said.



The focus of much of Nielsen's duties involves drilling and training horses such as Kennedy, a cap horse whose previous career involved running around harness racing tracks.



Cemetery ceremonial horses are washed and brushed until their coats have a bright sheen. Saddles and brass are buffed and polished until they shine like mirrors.



Ancient caissons that carry flag-draped caskets are cleaned and readied for a day of service.



As for the soldiers, Old Guard members' woolen uniforms are flawless and take hours to prepare, as each inch is inspected again and again.



Uniforms are pressed and ironed. Shoes and brass are polished and shined.



"In the winter, it can get pretty cold out there," Nielsen said. "In the summer -- it's no joke -- the summers get extremely hot. There will be heat indexes of 100 to 115 degrees."



Nielsen described his duty as rewarding. "We carry America's heroes to the final resting place," he said.



Soldiers in formation lead the procession. An Army band plays, and the unit marches to muffled drums.



The caisson passes, led by six horses, either black or white. The horses' harnesses jangle and the caisson wheels rumble through the hallowed paths of Arlington.



Bringing up the rear of the procession is Nielsen, leading Kennedy.



They pass in formation
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