What document changed the purpose of the civil war from one of states' rights to that of slavery?

That document was the Emancipation Proclamation that was issued on January 1, 1863.  This document not only freed the slaves but it also changed the main goal of the war itself.  At the beginning, Lincoln’s goal was to preserve the Union.  He was afraid that if he freed the slaves first, it would cause other states under the Union to break away.  Antietam enabled the him to finally issue the said proclamation and by doing so freed all slaves who were either in the North or the South.