Final answer:
As a result of the Battle of Antietam, Lincoln issued the preliminary Emancipation Proclamation, which freed slaves in rebellious Confederate territories and shifted the war aims towards the abolition of slavery.
Step-by-step explanation:
As a result of the Battle of Antietam, President Abraham Lincoln was able to issue the preliminary Emancipation Proclamation on September 22, 1862. The proclamation declared that slaves in those Confederate areas still in rebellion against the United States would be "henceforth and forever free."
It was framed as a necessary war measure for the preservation of the Union and had profound implications for the war's direction, making it more explicitly a fight against slavery. The proclamation also endorsed the voluntary colonization of freed slaves and recommended that loyal slave states gradually abolish slavery.