That is the popular story in our history books. But, so much more has come to light that has reconfigured the thinking of that period of history.
We know that the E.P. was strickly a political move by Lincoln to keep England at least out of the War. Making the war aim to be for now "freeing the slaves" rather than "Preserving the Union" would have certainly countered this popularist movement in England which the ex-Mayor of London was making mention of.
We also know that Lincoln was in talks with England over a new treaty for the Slave trade on the High Sea, which was originated by England.
By Confederate Constitutional Law new slaves could only be imported into the Confederate States from only one country, The United States.
Winning or losing at the Battle of Sharpsburg (Antietam) had little, or I would say nothing to do with the grander political concerns of Lincoln in issueing the E.P.. Winning the War was his main concern and he said so in just those words, That if he could win the War by freeing the slaves he would, if he could achieve his aims by freeing some some slaves and leaving some in bondage, he would do that. But if he could preserve the Union by leaveing all of the States in slavery, he would do that too.
This letter demonstrate that he was very close to losing the popular opinion battle for the English people. If England had entered the war on the side of the Confederacy there would been the end of Lincoln's Union of all the States.