Joseph talks about the potential effects of emancipation.
Joseph Smith, Jr. Letter to Oliver Cowdery, in Messenger and Advocate, April 1836, 289
I am aware, that many who profess to preach the gospel, complain against their brethren of the same faith, who reside in the south, and are ready to withdraw the hand of fellowship because they will not renounce the principle of slavery and raise their voice against every thing of the kind. This must be a tender point, and one which should call forth the candid reflection of all men and especially before they advance in an opposition calculated to lay waste the fall States of the South, and set loose, upon the world a community of people who might peradventure, overrun our country and violate the most sacred principles of human society,—chastity and virtue.