It enabled Abraham Lincoln to issue the Emancipation Proclamation, by giving the Union enough confidence (The battle had forced the Confederate troops to withdraw, making it technically a Union victory) to change the focus of the war from bringing the wayward states of the South back into the fold, into a war to destroy slavery in America. One major consequence of the Emancipation Proclamation was that the European powers that were interested in helping the CSA were discouraged from interfering, given that their populations had a severe distaste of slavery, and a war to support slavery would not be taken well by their people.
