Final answer:
After the First Battle of Bull Run, both the Union and Confederacy recognized that the Civil War would be neither short nor easy, shattering early illusions of a quick resolution and forcing both sides to prepare for a protracted and costly conflict.
Step-by-step explanation:
Both the Union and Confederacy realized that the Civil War would not be a short or easy conflict after the First Battle of Bull Run. Initially, both sides entered the war with misconceptions: the North believed it could quickly subdue the southern rebellion, while the South thought it could rapidly secure independence. The battle at Bull Run shattered this naïveté. The Union recognized the formidable resolve of the Confederacy, and the Confederates underestimated the Union's determination to preserve the nation. Additionally, the battle prompted Northern abolitionists to demand slavery's abolition become a central aim of the war effort, adding a moral imperative to the Union cause.
The Union's early optimism was punctured by the defeat at Bull Run, which sent shock waves through the populace and government. It became clear that the war would not be won only by superior numbers or resources, but would require prolonged and bloody engagement. Both sides then had to brace themselves for a grueling and extended conflict that would demand significant sacrifices.