THE FLAG WAS THE FIRST THING I NOTICED when I pulled into the parking lot at Fort McHenry on the Baltimore Harbor. I was there to run the tidy one-mile loop along the perimeter of the National Park Service site where 19th-century American forces famously repelled the British. I expected water views and the breeze. I hadn’t anticipated the flag.

It was early spring, but the events of Jan. 6, 2021, were still fresh in my mind. Like millions of Americans, I watched the insurrection at the Capitol gain momentum during a stunningly fraught series of afternoon confrontations. In the days that followed, among the video clips playing over and over on my news feed was that of a rioter beating a Metropolitan police officer with a pole that on its other end carried the American flag. I was, I realized, unsure what the flag meant, to those who display it and to those who find themselves standing beneath it.

At Fort McHenry, the flag flies so high that it nearly upstages the five-sided 18th-century fortress that, from above, resembles a star. A closer look reminded me what the place is all about. Flapping with the breeze above Fort McHenry is not the banner we know today, with its 13 stripes and 50 stars. The Park Service displays the 14-star flag that flew there during the Battle of Baltimore in September 1814 — an emblem of a nation still in formation and engaged in a pitched fight to settle, finally, its independence from Britain.

Among those present as the battle raged was Francis Scott Key, a prominent Maryland lawyer and amateur poet. Key had a distinct vantage point as the British bombarded Fort McHenry. Held temporarily as a prisoner of war, he witnessed munitions raining down – “bombs bursting in air.” When the strife settled, he discovered the U.S. flag – “the star-spangled banner” – flying above the fort and signaling that the British assault had failed.

Key penned a poem in the hours that followed, one that honored the flag of 1814 as a symbol of the nation’s valor and best ideals: “the land of the free and the home of the brave.” Set to a popular tune, Key’s words became the national anthem, though only many decades later, in 1931.

Encountering the flag that Key, a onetime slaveholder, celebrated unsettled me. Even as prominently as it waves over Fort McHenry, it is not a universal symbol. The third stanza of his poem, intending to honor the flag flying over Baltimore Harbor, is an admission that his world included enslaved men: “No refuge could save the hireling and slave, from the terror of flight, or the gloom of the grave: And the star-spangled banner in triumph doth wave, O'er the land of the free and the home of the brave.”

Key divided Americans between the free and the unfree. And among British forces, as he noted, were members of the British Corps of Colonial Marines, formerly enslaved men brought on to help defeat the U.S. Key saw the War of 1812 as, in part, a conflict between formerly enslaved men and American slaveholders. Likely his sympathies rested with the latter, and Key’s America did not include those held in bondage. This makes his anthem controversial still in our own time.

With that backstory embedded in its lyrics, Key’s anthem and the flag it sought to honor share a symbolic potency that has endured over two centuries. They both carry the power to divide and to unite us.