First Battle of Bull Run
Also called: First Battle of Manassas, First Manassas
Marching on Richmond, the Union army was beaten by the Confederates on Bull Run creek near Manassas, Virginia, and fled back to Washington; hopes of a short war collapsed.

Kurz & Allison · Public domain · Source
Pushed by the cry of “On to Richmond!”, Irvin McDowell’s Union army of about 35,000 attacked the Confederates under Beauregard and Joseph Johnston along Bull Run on 21 July 1861; spectators, congressmen among them, came out from Washington to watch. The Union pressed forward in the morning, but when Thomas Jackson’s brigade held on Henry Hill a fellow general is said to have cried, “There stands Jackson like a stone wall” — the origin of the name “Stonewall”.
When Confederate reinforcements arriving by rail counter-attacked, the green Union troops broke and fled to Washington. Casualties were about 2,900 Union and 2,000 Confederate. Both sides realised the war would be long and bloody, and Lincoln had George McClellan rebuild the army.