If I remember correctly, the main objective (besides the fact that it was the right thing to do) was to create chaos and destabilize the South so it would have to surrender. The South was virtually powerless without its slaves; that was the main reason the South did not want to give up slavery in the first place. The South's main source of income was from tobacco and cotton, and the slaves were the backbone of the production of these crops. In some parts of the South, blacks even outnumbered whites.
The emancipation proclamation was a tactical tool in the war with the South. It actually only freed the slaves in the South and was supposed to foment insurrection of the slaves against the South. After the war it was extended to the northern slaves as well, because from a constitutional standpoint a national law must be equally applicable to all citizens and cannot be applied differently based on location.
To signal to Britain and France that they could no longer help the Confederates without lookig pro-slavery themselves.
to prevent britain from siding with the south
One of Abraham Lincoln's most famous acts was issuing the Emancipation Proclamation in 1862, which declared that all slaves in Confederate territory were to be set free. This proclamation played a significant role in the abolishment of slavery in the United States.
Lincoln is famous for holding the Union together during the Civil War, for issuing the Emancipation Proclamation, and championing the 14th Amendment.
Abraham Lincoln was a president of the United States of America. He was president of the Union during the Civil War, and wrote the Emancipation Proclamation.
Because after Lincoln turned it into a war on slavery (by issuing the Emancipation Proclamation), it would have made them look pro-slavery themselves.
The British had to give up their plans to grant recognition to the Confederacy and send military aid, for fear of looking pro-slavery.
your butt
The answer the question is probably looking for is "the Emancipation Proclamation", which kept Britain and France (both of which were anti-slavery, but also more or less anti-Union) from officially recognizing and possibly supporting the Confederacy.
* to clarify the status of the fugitive slaves,* to solve the Union's manpower woes,* to keep Great Britain out of the conflict,* to maim and cripple the Confederacy by destroying its labor force,* to remove the very thing that had caused the war, and* to break the chains of several million oppressed human beings and right America at last with her own ideals
He hoped to do so by issuing the Emancipation Proclamation, but the Northern public did not react with much interest. The chief impact of the Proclamation was to make it impossible for Britain and France to aid the Confederates without looking pro-slavery themselves.
Kept Britain and France from helping the Confederates, by issuing the Emancipation Proclamation, which presented the war to the outside world as a moral crusade, making it unethical for free nations abroad to aid the South.
By issuing the Emancipation Proclamation - turning it into an Abolitionist war, so that free nations abroad could not aid the South without looking pro-slavery themselves.