On Memorial Day, Americans turned their thoughts to the nation's fallen heroes and one place above all: Arlington National Cemetery. It's the most famous and beloved of our nation's cemeteries, attracting more than 4 million visitors a year.

I once served at Arlington with the U.S. Army's Old Guard, which performs military funeral honors there. In a sign of our national fascination with Arlington, the single most frequent question I get when I speak around the country is about my service with the Old Guard. Likewise, Arkansans consistently rank the cemetery as the highlight of their visits to Washington.

But the cemetery is running out of space - and fast. If we don't act soon, the future of this national jewel will be at risk.

Arlington was born in the tragedy of the Civil War. The cemetery was originally Gen. Robert E. Lee's plantation. Lee made his fateful decision to resign from the U.S. Army and join the Confederacy after a sleepless night in what's now known as the Arlington House in the cemetery. The Union shortly thereafter seized the plantation, which occupied the critical high ground above Washington.

In 1864, as the Union war dead piled up in Washington, Union Quartermaster General Montgomery Meigs proposed that 200 acres from Lee's plantation be converted into a cemetery. As a further rebuke, Meigs ordered the construction of a burial vault in Lee's garden, where the remains of 2,111 unknown Union dead were interred.

While Lee made no attempt to reclaim his old plantation, his oldest son did, prevailing in the Supreme Court and then selling ownership back to the federal government. And, gradually, the Arlington we know today took shape, and the land it occupied expanded. As the nation's wounds from the Civil War healed, Arlington reflected that healing, accepting Confederate soldiers for burial, as well as reinternments from the Revolutionary War and the War of 1812.

After World War I, attention turned once again to Arlington as America buried its 116,000 war dead, including many unknown remains. Inspired by ceremonies in London and Paris honoring unidentified soldiers, many Americans began to advocate for our own shrine of unknowns, which was dedicated in the new Memorial Amphitheater in 1921. Yet by 1980, the demand for burial space was so great that the cemetery began building one of nine columbaria to hold some 45,000 cremated remains. Today, the cemetery is the final resting place for 400,000 souls.

But Arlington is not just a tourist site; it's an active cemetery with dozens of funerals daily, in all their majestic solemnity. It's a moving sight. A casket on a horse-drawn caisson rolls slowly toward the gravesite, followed by dozens of servicemen in immaculate dress uniform. The casket team lowers the remains onto the mock-up and stretches the flag out and level directly over it. After a brief service, the family rises, the officers present arms, and a seven-man team fires a three-volley salute - three sets of seven shots. Then, a lone bugler plays "Taps." Once the family sits, the casket team folds the flag, which the officer in charge presents to the next of kin.

Burial in Arlington is itself a healing process - and one the families of the fallen deserve. That's why it's so important that we preserve Arlington for the next generation. But if current trends continue, the cemetery will run out of space in just 25 years. In other words, veterans of every conflict from the Gulf War onward cannot hope to be buried there, even if they earn the nation's highest decoration, the Medal of Honor.

In August, the cemetery will complete its Millennium Project, which will add 27 acres and 28,000 burial sites, extending the cemetery's capacity to 2041. Another undertaking, known as the Southern Expansion project, began in 2014 and remains in the planning stages. It would add 40,000 to 60,000 burial sites, mostly in the area surrounding the nearby Air Force Memorial, and extend the cemetery's capacity for 10 years more.

But to do this, the cemetery must buy land from Arlington County and Virginia, as well as address transportation challenges with these authorities. Congress must support these efforts, particularly with funding for the land acquisitions, and look into whether the cemetery should expand to adjacent federal land on behalf of our nation, our fallen warriors, and their families.

As the war that created Arlington broke out, President Abraham Lincoln called for succor upon "the mystic chords of memory, stretching from every battlefield, and patriot grave, to every living heart and hearthstone, all over this broad land." Those chords stretch still from Arlington National Cemetery and ought to be swelled again by patriot graves yet to come.

U.S. Sen. Tom Cotton (R., Ark.) is a former U.S. Army captain. He wrote this for the Washington Post.