Fort Barrancas mostly by enslaved African-American workers. January 8, 1861, more than three months before the American Civil War officially started at Fort Sumter, South Carolina, a company of 50 U.S. Army soldiers stationed at Fort Barrancas, under the command of John H. Winder fired upon a militia. This militia was Florida state troops under Colonel William Henry Chase demanding that the U.S. troops surrender the fort. Lieutenant Adam J. Slemmer, acting commander in Winder's absence, had the troops fire shots meant to repel the militia. Lt Slemmer knew that Fort Pickens was easier to defend, so he spiked the guns at
Fort Farnsworth now comprising the county was retroceded to Virginia in a July 9, 1846 act of Congress that took effect in 1847. Most of the county is hilly, and at the time, most of the county's population was concentrated in the city of Alexandria, at the far southeastern corner of the county. In 1861, the rest of the county largely consisted of scattered farms, the occasional house, fields for grazing livestock, and Arlington House, owned by Mary Custis, wife of Robert E. Lee. Following the surrender of Fort Sumter in Charleston, South Carolina, on April 14, 1861, the new American President,