The American Civil War, initiated in April 1861 by those resorting to violence to prevent abolition of slavery, was in its third year when President Lincoln made his famous Emancipation Proclamation:
…all persons held as slaves [within the rebellious states] are, and henceforward shall be free.
The exact phrasing, developed and revised over many prior months, targeted only the “rebellious” states; those who quit the Union with intent to preserve slavery (as detailed by their secession papers, which clearly listed keeping slaves as a most pressing concern). Thus a notable exception was granted for other states, such as those already won by Union forces and no longer in the rebellion. His Proclamation neither applied to them nor those states remaining loyal to the Union, even bordering on Confederate territory. In other words slaves were proclaimed free for those states still in rebellion, areas intent on dissolving a Union to preserve slavery.
The relevance of Lincoln’s words to states in rebellion obviously pivoted on ability of the Union to reassert its authority within. This essentially changed the tone of conflict, as a mission of liberty from terror was proclaimed. After January 1, 1863 any Union recapture of territory meant Northern troops were said to be bringing freedom to Americans, ending a Southern reign of violence against those who had dared speak about abolition.
Moreover, this Proclamation literally allowed liberated territory slaves to join the Union in its fight against rebellion. Hundreds of thousands of black men, freed from the injustices put upon them by Confederates, signed up to serve in federal Army and Navy forces to end white police state terror that secession had intended to preserve.