Final answer:
President Lincoln announced the Emancipation Proclamation after the Union's strategic success at the Battle of Antietam in 1862. This timing allowed Lincoln to capitalize on the military gains and framed the proclamation as both a war measure and a step towards universal abolition, altering the Civil War's objectives profoundly.
Step-by-step explanation:
President Abraham Lincoln chose to announce the preliminary Emancipation Proclamation after the Battle of Antietam to utilize the moral and strategic advantage of a Union victory. By issuing it following a significant military engagement, Lincoln hoped to strengthen the Union's moral position and add pressure on the Confederate states by making abolition a war aim. The proclamation also aimed to undermine the Confederacy militarily by stripping away its slave labor force and deterring foreign intervention from nations like Britain, which had strong anti-slavery sentiments.
Antietam provided a crucial moment as it represented a strategic shift and was deemed enough of a victory for Lincoln to assert his authority and introduce the Emancipation Proclamation. It, therefore, served as both a war measure to debilitate the Confederacy's ability to sustain its war effort and a moral stance against slavery, effectively redefining the Union's goals in the Civil War. This decisive move further reflected Lincoln's evolving perspective on emancipation as an essential component for preserving the Union and winning the war.