Emancipation Proclamation is a major decision that 
USA can take, when they are winning or have won the American Civil War against 
Confederate States of America.
It requires that USA has at least 35 prestige and have occupied at least 40% of CSAs land, which mean that the war has to go relatively well. Alternatively it can be taken after the civil war, when CSA does not exist.
It will add Afro American as an accepted POP, outlaw Slavery and decrease militancy and consciousness for all Yankee POPs by 5. On top of that it increases the relation with all other Great Powers by 30.