The Arlington Line was a series of fortifications erected in present-day Arlington County, Virginia, to protect the City of Washington during the American Civil War. Just across the Potomac River from the Union capital, Confederate Virginia was a major Union concern when the war began. In May 1861, federal troops seized much of present-day Arlington County (which borders the Potomac), and immediately began constructing an extensive network of fortifications to protect Washington. That network eventually became known as the Arlington Line. The pace of construction accelerated after the Confederacy defeated the Union at the First Battle of Bull Run in July 1861.