We call it First Manassas and the Yankees call it the First Battle of Bull Run. It was the first real battle of the War Between the States (It was not a Civil War - there was nothing civil about it).
As with most battles, this one was more for political reasons than military. Both sides wanted to end the war as soon as possible and McDowell wanted to invade Virginia, crush the Confederates and push on to Richmond, now the Confederate Capital.
The Yankees gained an early advantage because it had numbers on its side, but on a small hill called Henry House Hill, Thomas J. Jackson stood his ground and earned the nickname "Stonewall". Just in time Johnston's army arrived to reinforce Beauregard, and the day was saved for the Confederates.
Retreat was called and the Union soldiers began to race to the rear as the Confederates shot at them.The retreat turned into a rout as the terrified soldiers fled in panic, with the situation being made much worse by the presence of hundreds of sightseers with picnic baskets in hand...and it was 150 years ago today!