Final answer:
The Battle of Antietam led to Lincoln's Emancipation Proclamation, shifting the war's purpose toward ending slavery and deterring European intervention for the Confederacy.
Step-by-step explanation:
The sacrifice of the 23,000 soldiers who were killed or wounded at the Battle of Antietam resulted in neither a clear Union victory nor a decisive Confederate defeat. However, it led to President Abraham Lincoln issuing the Emancipation Proclamation, which was a significant shift in the moral objective of the war from solely restoring the Union to also eliminating slavery.
Although the proclamation had many exemptions, it profoundly impacted the war's direction, ensuring European powers like Britain and France did not recognize or support the Confederate states, thereby removing the possibility of their intervention in favor of the Confederacy. Despite the heavy casualties at Antietam and subsequent battles, such as Fredericksburg, the Confederate army continued to fight on until key defeats, like Gettysburg and Vicksburg, led to a turning point in the war that favored the Union.
Emancipation Proclamation and its consequences had broader implications, including altering international perceptions and inhibiting Confederate hopes of securing alliances. It did not, however, lead to the Confederate troops abandoning their posts in large numbers immediately. The struggle continued for several more years, with pivotal battles shaping the course of the Civil War, ultimately culminating in the Confederacy's defeat.