THE DAY AFTER Virginia seceded from the Union in May 1861, President Lincoln sent federal troops to occupy Alexandria. At their head in the dawn's early light strutted the dashing and endearing Col. Elmer Ellsworth, 24, the North's most colorful soldier.

Less than an hour after he had set foot on Southern soil, Ellsworth lay dead in the folds of a captured Confederate flag, the first officer killed in the Civil War.

Ellsworth's "martyrdom" sent the North into paroxysms of grief and rage surpassed only by the assassination of Lincoln four years later. James Jackson, the hotel keeper who slew Ellsworth and was himself instantly shot down, became an equally mourned and celebrated hero of the South.

This dramatic early episode of the Civil War is vividly retold in a new exhibit at Alexandria's Fort Ward Museum. The display has all the right stuff, including the dapper little Ellsworth's uniform with the fatal hole in the left breast and a fragment of the flag for which Jackson died. It also has a masterful narrative by the museum staff that says more in fewer words than almost any text this side of the Gettysburg Address.

No dramatist could haved scripted a scene more suited to that romantic and sentimental time. Ellsworth's death was the more affecting because he probably had been seen in the flesh by more Americans than any other living man, having led his crimson-clad Zouaves on a tour of almost every major Northern city. He had been so lionized by the Yankee press that his name and face were known wherever newspapers circulated.

In his last letter to his parents, written only hours before his death, Ellsworth said, "Whatever may happen, cherish the consolation that I was engaged in the performance of a sacred duty."

Lincoln and his family long had regarded the irrepressible Ellsworth almost as a son, and wept over him as he lay in state in the East Room of the White House.

Jackson, 37, was known only as manager of the Marshall House, a second-rate hotel at King and Pitt streets, and as perhaps the second-best pugilist in his native Fairfax County. But when he had commissioned a city sailmaker to sew his 14- by 24-foot Confederate flag, Jackson told him, "Whoever should attempt to remove it would have to pass over {Jackson's} dead body." Southerners saw him as a fearless patriot defending his home and flag against invading tyrants.

Jackson was asleep when Ellsworth and several of his men went storming up the stairs of the Marshall House to seize the flag, and faced them alone when the "boy colonel" came down with the rebel banner draped over his arms. Jackson's point-blank shotgun blast was centered on a patriotic medal pinned to Ellsworth's chest, and drove it into his heart. He "dropped forward with . . . heavy, horrible, headlong weight," wrote a newspaper reporter who was standing behind Ellsworth. An instant later Jackson was shot in the head by Cpl. Frank Brownell, who was awarded the Congressional Medal of Honor. In succeeding weeks souvenir hunters all but whittled away the hotel's woodwork, whose fragments were venerated very like slivers of the True Cross.

Remember Ellsworth and Remember Jackson became watchwords and battle cries in the opening engagements of the war, until the carnage reached such heights that there were too many dead heroes for the public to keep in mind.

Although done on a shoestring, the Fort Ward exhibit is full of deft touches, illuminating vignettes and telling details. The enormous amount of information it contains is so well-written and cleverly compressed that there are no signs of strain. It's a model of what can be done without multimedia gimmickry.

COL. ELMER ELLSWORTH, THE FIRST TO FALL -- Through September 1991 at the Fort Ward Museum, 4301 W. Braddock Rd., Alexandria. 838-4848. Open 9 to 5 Tuesday through Saturday and noon to 5 Sundays. Good wheelchair access.