B. It was the first major battle between the North and South.
The First Battle of Bull Run, or the First Battle of Manassas, was the first major land battle in the American Civil War.
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Reasons why it cannot be A, C, or D:
A. The Union did not win this battle.
C. Grant and Lee met in the Mexican-American War.
D. Jackson was promoted to Brigadier General in June 1861, after Harpers Ferry, then a Major General in October of that year.