What was the cause of the First Battle of Bull Run?

Answer 1

The Union planned to cut off the railroad at Manassas in Virginia and move on to Richmond, the Confederate capital. The Union never made it to Richmond, however, because a Confederate spy, Rose Greenhow, a Washington socialite who hob-nobbed with the politicos heard about the Union's plans and told the Confederacy.

In the end the Union retreated back to Washington (The Great Skedaddle) and the Confederacy, instead of following them there and taking over Washington spent time picking up the items the Union soldiers discarded.

The result being that both sides realized that their armies need more training.

As an aside, there are sometimes different names for the same battles because the Union would sometimes name a battle after the nearest creek/body of water and the Confederacy after the nearest town. Hence, Bull Run/Manassas...:-)

Answer 2

Bull Run (known as Manassas on the Confederate side) was the first major battle of the War. It had been months since the attack on Sumter, and in the North there were rising concerns that the more time passed, the stronger the Confederate Army would become. The Union wanted to hurry up and engage in a battle, as did the Confederates. At that point, both sides believed one large decisive battle would settle things - they could not forsee the long war to come. As it turned out, the battle was highly unorganized on both sides, and set the stage for a long conflict. The railroads and roads intersecting in Manassas, and the proximity to both capitols, made it an ideal location to battle.

Answer 3

General P.G.T. Beauregard, was directed to "assume command of the Confederate troops on the Alexandria line."

"Although the position a the time was strategically of commanding importance to the Confederates, the mere terrain was not only without natural defensive advantages, but, on the contrary, was absolutely unfavourable. It's strategic value was that, being close to the Federal capital, it held observation the chief army being assembled near Arlington...for an offensive movement against Richmond."
P.G.T. Beauregard

"Early in June General Robert Patterson was pushing his column against Harper's Ferry, and on the the 3rd of that month General Irwin McDowell was called upon by General Winfield Scott to submit " an estimate of the number and composition of a column to be pushed toward Manassas Junction and perhaps the Gap, say in 4 or 5 days, to favour Patterson's attack upon Harpers Ferry."
James B. Fry

The Confederates were defending their capital, Richmond, against a Federal attack. The Federals were attempting to attack simultaneously at Harper's Ferry and advance on the Confederate capital Richmond before the period of time that their soldiers enlisted for expired.