Final answer:
The First Battle of Bull Run was an ominous event signifying a significant Confederate victory, shattering the Union's morale, and signaling a prolonged and arduous Civil War.
Step-by-step explanation:
The Bull Run event was an ominous one for President Lincoln, the Union, and the Civil War in general because it signaled a significant Confederate victory. At the First Battle of Bull Run, both Union and Confederate forces anticipated a quick resolution to the conflict. However, the Union's defeat shattered this hope, demonstrating that the Confederacy was a formidable opponent and that the war would be neither brief nor easy. The loss at Bull Run ruined Northern morale and dispelled any illusions of an inevitable Union victory. This battle marked the beginning of President Lincoln's arduous search for a competent general who could lead the Union to victory.
Moreover, the victory bolstered the Confederacy's resolve and proved that they were committed to their cause, deeply entrenching the opposing sides for a prolonged conflict. The impact of the Battle of Bull Run reinvigorated calls for emancipation as a war aim, with prominent figures like Frederick Douglass arguing that the Union should dedicate itself to destroying the institution of slavery that the Confederates fought to defend.