Bull Run, First Battle of, or First Manassas, July 21, 1861, the first major encounter of the American Civil War. It was fought near Bull Run, a creek in Virginia about 30 miles (48 km) southwest of Washington, D.C. The Confederate army under Generals P. G. T. Beauregard and Joseph E. Johnston lay near the town of Manassas in position to defend Richmond. Union troops under General Irvin McDowell advanced to attack them.

Both armies were small and inexperienced. With the Union army were many spectators who expected to see their troops win an easy victory. But the Northern forces soon began to fall back and the retreat turned into a rout. This quick victory bolstered the South's confidence and awakened the North to its need for a large army of well-trained troops.