Final answer:
The Emancipation Proclamation, issued by Abraham Lincoln on January 1, 1863, freed the slaves in the Confederate states in rebellion against the Union. While it did not immediately liberate all enslaved people, it marked a significant move towards the abolition of slavery and altered the purpose of the Civil War.
Step-by-step explanation:
The Emancipation Proclamation
The document that freed the slaves in the Confederate states that were rebelling against the Union was the Emancipation Proclamation. Issued by President Abraham Lincoln on January 1, 1863, during the American Civil War, it declared "that all persons held as slaves" within the rebellious states "are, and henceforward shall be free." This measure took effect in areas still in rebellion and not yet under Union control, thus it didn't immediately free all enslaved people, but paved the way for slavery's eventual abolition.
Lincoln considered the abolition of slavery a military necessity, essential for the preservation of the Union. Importantly, he issued the Emancipation Proclamation under his powers as commander-in-chief, which minimized legal challenges as it did not apply to slave states that had not seceded or areas under Union control at the time. While the proclamation was limited in its immediate effectiveness, it was a crucial step towards ending slavery, and it significantly changed the war's focus to a fight for human freedom.
Black southerners, most of whom were enslaved, overwhelmingly supported the Union. The presence of Union troops in the South as the war progressed led to many slaves fleeing plantations and seeking freedom—a movement that grew in power and eventually changed the social landscape of the American South.