One of the Civil War’s seminal moments occurred just across the river in Alexandria, Va. 

It was May 24, 1861, and President Abraham Lincoln ordered Union troops to occupy the recently seceded area of Northern Virginia. A rabid secessionist, James W. Jackson, had already hoisted a huge Confederate flag above the Marshall House, where he served as the innkeeper. 

Lincoln could see the massive flag from the second story of the White House, and the story goes that his close friend Col. Elmer Ellsworth — the young commander of the First New York Fire Zouaves — offered to take it down.

Hours later, Ellsworth would be dead. 

His place as the first Union officer killed in the Civil War would catapult Ellsworth to the forefront of the nation’s newspapers and make him a martyr figure for the North. And in “The Death of Ellsworth,” a new exhibit at the National Portrait Gallery, this oft-forgotten colonel again takes center stage.

“Civil War buffs know this story, but others don’t,” said James Barber, National Portrait Gallery historian and curator of the exhibition. “It’s a national story in that Ellsworth was the first Union officer to be killed in the war. It’s also a local story. … And for me, this is really the start of the Civil War. This is the point, for me, where there is no turning back.”

Ellsworth had led his troops into the city, Barber said, and was afraid that his notoriously unruly firefighter soldiers — as famous for setting fires as for putting them out — might get out of hand once they saw the stars and bars flying over the Alexandria skyline. 

Ellsworth and a few men trekked over to the Marshall House and commandeered the flag. Then, in a moment that would come to dominate scenes depicted on Northern mementos like those showcased in the exhibition, Jackson killed Ellsworth as the colonel walked down the inn’s stairs with the flag in his hands.

“And Ellsworth’s coming downstairs, he’s just taken the flag down from atop the 30-foot pole,” Barber said. “And Jackson was just an ardent secessionist, a radical secessionist. He was sort of a mean man, too, and some say he had a bit of a death wish. He shoots Ellsworth — the guy without the gun. And the second barrel, he misses. A wiser person would dispatch the guy with the gun first. So then Francis Brownell dispatches Jackson with a rifle, and for good measure, uses the bayonet.”

The house, Barber said, instantly became a mecca for sightseers.

“It becomes this tourist attraction,” he noted. “People begin taking away souvenirs, carving up the flagstaff, pieces of the flag, taking a little bit of blood-stained flooring.”

The exhibit, though small, features souvenirs from the house as well as paintings, photographs and other relics and commemorative pieces. In the alcove, the rifles of Brownell and Jackson flank a painting, “The Death of Ellsworth,” by Alonzo Chappel. 

Visitors can also view sheet music, envelopes and even a vase depicting Ellsworth’s violent end. For Barber, one of the most interesting items on display is the photograph of Jackson, Ellsworth’s killer.

“Now, I’ve seen a lot of faces at the gallery here over the years and a lot of quotes that went with them, but I don’t know when one quote has matched a face like this,” Barber said. “‘His face was remarkable in its expression. Grim, stern, obstinate determination was stamped emphatically on every feature.’ Look at that sneer — he’s just not the type of person you’d want to meet, especially if he had a grudge.”

For Northerners in 1861, it would be almost unimaginable that Ellsworth’s name might be forgotten by future generations. This new exhibit, Barber said, gives visitors to the gallery a chance to look into the life — and afterlife — of Ellsworth.

“It was widely known — people named their children after Ellsworth, regiments were named the Ellsworth Avengers — during the Civil War and after,” Barber said. “And then as time passes, it becomes more and more of a local story. … If Alexandria makes the history books for the Civil War, it is usually in relation to the Ellsworth incident.”

“The Death of Ellsworth” is on display at the National Portrait Gallery through March 18, 2012.