Most people know the Old Guard as the Army regiment that protects the Arlington Nation Cemetery and its Tomb of the Unknown Soldier. But it has a long combat history too.

Before he was a senator, Tom Cotton served in the storied regiment. He unearths its forgotten past in “Sacred Duty,” published earlier this May by the William Morrow imprint of HarperCollins.

His own time in the Old Guard, between 2007 and 2008, didn’t include the unit’s most visible duty.

“Most people assume I guarded the Tomb. I joke that I was not a Tomb guard because I was an officer and officers cannot be trusted to execute such a sensitive duty,” the Arkansas Republican writes.

The truth has little to do with his rank. The guard is a fully trained infantry unit, and its duties extend beyond the ceremonial, even today.

It all began back in the 18th century, when the regiment fought to drive British forces out of United States territory and into Canada, taking part in the successful Battle of Fallen Timbers in 1794.

“Fallen Timbers marked the first military triumph of today’s Old Guard, and the regiment commemorates the campaign every day. On their left shoulder, Old Guard soldiers wear a black-and-tan ‘buff strap,’ which mimics the shoulder straps on the rucksacks carried by [Major Gen. Anthony] Wayne’s troops,” Cotton writes.

These days soldiers earn the buff strap by passing the marching and uniform tests required to conduct funerals in Arlington National Cemetery.

The Old Guard would go on to serve in other campaigns across North America and around the world, including in the Philippines, but as Cotton explains, it was their service during the Mexican-American War that earned the forces their nickname.

It’s also why Old Guard soldiers to this day march with bayonets fixed.

“The 3rd Infantry led a bayonet charge up the hill, killing or capturing the enemy forces, then turning the Mexican artillery on other Mexican positions,“ he writes of the regiment’s pivotal role in the Battle of Cerro Gordo in April 1847.

Who was the young Army captain who suggested that the Americans, including the future Old Guard, could outflank the Mexican forces at Cerro Gordo despite the extremely complicated terrain? That would be Robert E. Lee.

It is, of course, the site of Lee’s estate — which became a Union stronghold after Lee sided with the Confederacy — that is now home to Arlington National Cemetery.

And it is at Arlington where the Old Guard is responsible for honoring and remembering deceased American troops and conducting funerals for the recently departed.

“Not too many Old Guard soldiers understand these things when they report to Fort Myer and start their training; they mostly know about the regiment’s modern ceremonial mission,” Cotton writes. “But none can fail to learn.”

More recently, the Old Guard has served overseas in Iraq and Djibouti, but it is the ceremonial duties that get most of the attention. Even Cotton makes those solemn responsibilities the focus of his book.

In the course of his research, he got what appears to be extraordinary access to the unit’s preparations for funerals, the changing of the guard and other ceremonies.

“I spent a lot of late nights, weekends and holidays at the Tomb of the Unknown Soldier,” Cotton writes in the acknowledgements.

While Cotton is known as among the most predictably conservative members of the Senate, he largely sidesteps politics within the pages of “Sacred Duty.” He doesn’t hide his views on expanding the size and scope of the military, however.

“The Old Guard’s own history is intertwined with the tragic history of this land,” he writes. “War came to Arlington in the early days of the Civil War and on 9/11 — and in both cases, the Old Guard was present for duty. In between and ever since, our nation has buried its war dead in this soil. And while the Old Guard has borne this solemn responsibility for the last seven decades, it has dedicated even more years to the nation’s defense on the battlefield, a heritage that still echoes across the plains and hills of Arlington today.”