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.
4. One able general, Stonewall Jackson in the case of Bull Run, could seize the initiative and turn the tide of a battle.
5. Decisive pursuit was going to be a rare thing in this war. The Union army was routed from the field, but the Confederates, almost as disorganized in victory, was unable to pursue and destroy large portions of the Union force.
6. Railroads allowed rapid reinforcement of troops.
7. Both sides were determined to win, and no one battle would lead to the end of the war.
The Confederates were overjoyed in winning the first big battle and the Union was depressed. However, to an objective observer the main lessons drawn from Bull Run would have been that even with raw troops both sides had withstood heavy fighting for several hours, that the issue was in doubt for most of the battle and that the results of the battle were not decisive for either side. A long war, desperately waged by both sides, was clearly foretold by the first major fight of the Civil War.