Fort Lyon (usually Camp Lyon in Northern records) was a timber and earthwork fortification constructed south of Alexandria, Virginia as part of the defenses of Washington, D.C. during the American Civil War. Built in the weeks following the Union defeat at Bull Run, Fort Lyon was situated on Ballenger's Hill south of Hunting Creek, and Cameron Run (which feeds into it), near Mount Eagle. From its position on one of the highest points south of Alexandria, the fort overlooked Telegraph Road, the Columbia Turnpike, the Orange and Alexandria Railroad, the Little River Turnpike, and the southern approaches to the city of Alexandria, the largest settlement in Union-occupied Northern Virginia. The Huntington Station of the Washington Metro is located next to Fort Lyon's former hilltop site, which is commemorated by a historical marker at the north end of the station lot off King's Highway. Occupation of Arlington Before the outbreak of the Civil War, Alexandria County (renamed Arlington County in 1920), the Virginia county closest to Washington, D.C., was a predominantly rural area. Originally part of the District of Columbia, the land 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, new American president Abraham Lincoln declared that "an insurrection existed," and called for 75,000 troops to be called up to quash the rebellion. The move sparked resentment in many other southern states, which promptly moved to convene discussions of secession. The Virginia State Convention passed "an ordinance of secession" and ordered a May 23 referendum to decide whether or not the state should secede from the Union. The U.S. Army responded by creating the Department of Washington, which united all Union troops in the District of Columbia and Maryland under one command. Brigadier General Joseph K. Mansfield, commander of the Department of Washington, argued that Northern Virginia should be occupied as soon as possible in order to prevent the possibility of the Confederate