D. By promising an end to slavery in the confederate states
Explanation:
The Emancipation Proclamation changed the course of the civil war, from protecting the Union to redefining what they meant by freedom. Also, this meant an incentive that slaves in areas not occupied by the Union would be set free, but slaves in Union territory would remain enslaved.
Reference: Thompson, Kathleen. “The Meaning of the Emancipation Proclamation.” Civil Discourse, Civil Discourse, 18 Apr. 2018
