Elmer E. Ellsworth
Colonel Elmer Ephraim Ellsworth was a law clerk and United States Army soldier, best known as the first conspicuous casualty and the first Union officer killed in the American Civil War. Before his death, as tension built up toward the war, he had been the leader of a famous touring military drill team known as the Fire Zouaves and was a close personal friend of Abraham Lincoln. He was killed while removing a Confederate flag from the roof of the Marshall House Inn of Alexandria, Virginia, at Lincolns behest, as the flag had been visible from the White House as a defiant sign of the growing rebellion. Lincoln called him the greatest little man I ever met, and his body lay in state at the White House after his death. Following his death, Remember Ellsworth would become a Union rallying cry.
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Source: Wikipedia