Final answer:
Abraham Lincoln shifted the Civil War's aim toward ending slavery with the Emancipation Proclamation, issued on January 1, 1863, after the Union victory at Antietam.
Step-by-step explanation:
President Abraham Lincoln initially focused on preserving the Union rather than ending slavery at the outset of the Civil War in April 1861. As the war progressed and the need for a more definitive stance on slavery became apparent, Lincoln's policies evolved. The turning point came after the Union victory at Antietam in September 1862. On January 1, 1863, Lincoln issued the Emancipation Proclamation, which declared the end of slavery in rebelling states. This act was more symbolic initially, as it had limited immediate effect, but it was a critical step that redefined the aim of the Civil War to include the abolition of slavery. Subsequently, the Thirteenth Amendment was drafted to ensure the permanent end of slavery in the United States.