Fort Runyon was a timber and earthwork fort constructed by the Union Army following the occupation of northern Virginia in the American Civil War in order to defend the southern approaches to the Long Bridge as part of the defenses of Washington, D.C. during that war. The Columbia Turnpike and Alexandria and Loudon Railroad ran through the pentagonal structure, which controlled access to Washington via the Long Bridge. With a perimeter of almost 1,500 yards (1,400 m), and due to its unusual shape it was approximately the same size, shape, and in almost the same location as the Pentagon, built 80 years later.

Runyon was built immediately after the entry of Union forces into Virginia on May 24, 1861, on the land of James Roach, a Washington building contractor. Fort Runyon was the largest fort in the ring of defenses that protected Washington during the Civil War and was named after Brigadier General Theodore Runyon, commander of the Fourth Division of the Army of Northeastern Virginia during the First Battle of Bull Run. Union soldiers garrisoned the fort until its dismantling following the end of the Civil War in 1865. Today, no trace of the fort remains on the site, though a historical marker has been constructed by the Arlington Historical Society.

Before the outbreak of the Civil War, Alexandria County (renamed Arlington County in 1920), the county in Virginia 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 J.F.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 Army mounting artillery on the hills of Arlington and shelling government buildings in Washington. He also urged the erection of fortifications on the Virginia side of the Potomac River to protect the southern terminuses of the Chain Bridge, Long Bridge, and the Aqueduct Bridge. His superiors approved these recommendations, but decided to wait until after Virginia voted for or against secession.

On May 23, 1861, Virginia voted by a margin of 3 to 1 in favor of leaving the Union. That night, U.S. Army troops began crossing the bridges linking Washington, D.C. to Virginia. The march, which began at 10 p.m. on the night of the 23rd, was described in colorful terms by the New York Herald two days later:

There can be no more complaints of inactivity of the government. The forward march movement into Virginia, indicated in my despatches last night, took place at the precise time this morning that I named, but in much more imposing and powerful numbers.

About ten o'clock last night four companies of picked men moved over the Long Bridge, as an advance guard. They were sent to reconnoitre, and if assailed were ordered to signal, when they would have been reinforced by a corps of regular infantry and a battery....