The First Battle of Bull Run (the name used by Union forces), also known as the Battle of First Manassas (the name used by Confederate forces), was the first major battle of the American Civil War.

The battle was fought on July 21, 1861, in Prince William County, Virginia, just north of the city of Manassas and about 30 miles west-southwest of Washington, D.C. The Union's forces were slow in positioning themselves, allowing Confederate reinforcements time to arrive by rail. Each side had about 18,000 poorly trained and poorly led troops in their first battle. It was a Confederate victory, followed by a disorganized retreat of the Union forces.