Final answer:
The first major battle of the Civil War was the First Battle of Bull Run, fought near Bull Run Creek in Virginia on July 21, 1861. It was a Confederate victory and marked a sobering realization of the war's potential length and severity.
Step-by-step explanation:
The first major battle of the Civil War was fought near Bull Run Creek in Virginia. This conflict is known as the First Battle of Bull Run, or First Manassas, which took place on July 21, 1861. The battle was significant not only as the first major clash between the Union and Confederate forces but also because it shattered the illusion that the war would be short and decisive. It was believed that this battle might quickly end the conflict with a Union victory; however, Confederate forces proved their commitment to their cause by winning.
The Battle of Bull Run saw roughly eighteen thousand soldiers from each side engaged in combat. With significant casualties including 1,982 Confederate and 2,896 Union losses, the battle set the tone for the brutality and length of the Civil War which would continue for several more years. The encounter near Manassas also dispelled the notion that Southerners would not fire upon the American flag, revealing instead a deep-seated determination to fight for the Confederate cause.