Final answer:
President Lincoln announced the preliminary Emancipation Proclamation after the Union victory at Antietam in 1862, which declared all slaves in rebellious states to be free and shifted the Union's war aims to include the abolition of slavery.
Step-by-step explanation:
Following the Union victory at Antietam, President Abraham Lincoln announced the preliminary Emancipation Proclamation on September 22, 1862. This pivotal document declared that slaves in the Confederate states in rebellion were 'henceforth and forever free.' Lincoln considered the proclamation a military necessity and essential for the preservation of the Union. It also called for voluntary colonization and gradual emancipation in loyal states and recommended a Constitution amendment for compensated emancipation.
Despite exemptions in border states and certain Confederate territories, the proclamation signified a shift in the Union's war aims from merely preserving the Union to also abolishing slavery. The proclamation also made it unlikely for European powers to recognize the Confederacy, altering international perspectives on the Civil War.
Lincoln's stance on emancipation evolved throughout the war, culminating in the final Emancipation Proclamation that took effect on January 1, 1863.