Final answer:
The Emancipation Proclamation, issued by President Abraham Lincoln, declared that slaves in rebelling southern states were free, bringing a substantial shift in the Civil War towards the abolition of slavery.
Step-by-step explanation:
President Abraham Lincoln issued the Emancipation Proclamation which freed slaves in the southern states. Announced after the Union victory at Antietam, and coming into effect on January 1, 1863, it stated "all persons held as slaves within any State or designated part of a State, the people whereof shall then be in rebellion against the United States, shall be then, thenceforward, and forever free."
However, the proclamation had limitations: it did not apply to slave-holding border states or Union-occupied Confederate areas, and its enforcement depended largely on the presence of Union troops. Despite this, Lincoln's proclamation marked a significant change in the Civil War's purpose, transforming it increasingly into a war to end slavery.