On this day in 1861, Elmer Ellsworth was killed in Alexandria, Virgina, while he was removing a Confederate flag from a window in the Marshall House Inn. He is generally regarded as the first officer killed on duty in the Civil War.

From Wikipedia:

Ellsworth was born in Malta, New York, grew up in Mechanicville, New York, and lived in New York City. In 1854, he moved to Rockford, Illinois, where he worked for a patent agency. In 1859, he became engaged to Carrie Spafford, the daughter of a local industrialist and city leader. Carrie's father demanded that he find more suitable employment, so he moved to Chicago, to study law and work as a law clerk.

In 1860, Ellsworth went to Springfield, Illinois, to work with Abraham Lincoln. He studied law in Lincoln's office and helped Lincoln with his 1860 campaign for president. Ellsworth was only 5' 6" tall, but Lincoln called Ellsworth "the greatest little man I ever met." He accompanied Lincoln to Washington, D.C. in 1861.

Ellsworth became drillmaster of the "Rockford Greys", the local militia company, in 1857. He studied military science in his spare time. After some success with the Greys, he helped train militia units in Milwaukee and Madison. When he moved to Chicago he became colonel of Chicago's National Guard Cadets.

Ellsworth had studied the Zouave soldiers, French colonial troops in Algeria, and was impressed by their reported fighting quality. He outfitted his men in gaudy Zouave-style uniforms, and modeled their drill and training on the Zouaves. Ellsworth's unit eventually became a nationally famous drill team.

The Civil War broke into open warfare in April, and on April 15, 1861, Lincoln called for 75,000 troops to put down rebellion. Ellsworth helped recruit these soldiers: he raised the 11th New York Volunteer Infantry Regiment (the "Fire Zouaves") from New York City's volunteer firefighting companies, and returned to Washington as their colonel.

Ellsworth died shortly after returning to Washington. On May 24, 1861 (the day after Virginia's secession was ratified by referendum), President Lincoln looked out from the White House across the Potomac River, and saw a large Confederate flag prominently displayed over the town of Alexandria, Virginia.

Ellsworth immediately offered to retrieve the flag for Lincoln. He led the 11th New York across the Potomac and into the streets of Alexandria uncontested. He detached some men to take the railroad station, while he led others to secure the telegraph office and get that Confederate flag, which was flying above the Marshall House Inn. Ellsworth and four men went upstairs and cut down the flag. As Ellsworth came downstairs with the flag, the owner, James W. Jackson, killed him with a shotgun blast to the chest. Corporal Francis E. Brownell, of Troy, New York, immediately killed Jackson. Brownell was later awarded a Medal of Honor for his actions.

Lincoln was deeply saddened by his friend's death and ordered an honor guard to bring his friend's body to the White House, where he lay in state in the East Room. Ellsworth was then taken to the City Hall in New York City, where thousands of Union supporters came to see the first man to fall for the Union cause. Ellsworth was then buried in his hometown of Mechanicville, in the Hudson View Cemetery.

Thousands of Union supporters rallied around Ellsworth's cause and enlisted. "Remember Ellsworth" was a patriotic slogan: the 44th New York Volunteer Infantry Regiment called itself the "Ellsworth Avengers", as well as "The People's Ellsworth Regiment."

Relics associated with Ellsworth's death became prized souvenirs. The Smithsonian Institution and Bates College's Special Collections Library have pieces of the Confederate flag that Ellsworth had when he was shot—in 1894, Brownell's widow was offering to sell small pieces of the flag for $10 and $15 each. The New York State Military Museum and Veterans Research Center in Saratoga Springs has most of the flag itself and Ellsworth's uniform, showing the hole from the fatal shot. The Fort Ward Museum in Alexandria dedicates a section of their museum to Ellsworth, displaying the kepi he wore when he was killed, patriotic envelopes bearing his image, a piece of the Confederate Flag (on which Ellsworth's blood is visible), and the "O" from the Marshall House sign that a soldier took as a souvenir.

In 1862, the newly established county seat of Pierce County, Wisconsin, located in the undeveloped center of the county to settle the controversy between two established cities, was named Ellsworth, Wisconsin in his honor.

The 1876 Centennial Exposition in Philadelphia had a major exhibit devoted to Col. Ellsworth.

In addition, Ellsworth, Michigan, Ellsworth, Wisconsin, and Fort Ellsworth were named in his honor.