The First Battle of Bull Run, or First Manassas, was the first major battle of the Civil War. A Confederate victory, it gave lessons to those paying attention:
1. It amply demonstrated the hazards of sending half-trained troops into combat. Both the Union and Confederate armies were green, and it showed in clumsy battlefield maneuvers and an inability to coordinate attacks.
2. An early indication that it was much easier to defend and counter-attack than to launch an initial attack in the Civil War.
3. Rifled muskets were going to make this an exceptionally bloody war. 5,000 Union and Confederate casualties resulted from this battle, just slightly below the total American killed and wounded for either the entire War of 1812 or the entire Mexican War.