Final answer:
The North realized the Civil War would be a prolonged conflict after the First Battle of Bull Run, also known as First Manassas, which was the first major battle of the war.
Step-by-step explanation:
The North quickly realized that the Civil War was not going to end swiftly and easily after the first major battle, often referred to as the First Battle of Bull Run or First Manassas.
This initial battle shattered the early Northern confidence, as both sides witnessed the grim reality and ugliness of war, understanding it would be a long and arduous conflict.
The Union, despite its superior resources, faced a determined Confederate army that managed to secure a victory at this first significant engagement. While the Union had initially hoped for a quick resolution, the defeat at First Manassas made it clear that this would not be the case.