Answer:
At the outset of the Civil War, President Lincoln had not spoken out specifically on issues relating to slavery, but on the contrary, had established that abolition of slavery was not one of the mainstays of the Union, but the maintenance of national unity.
Now, as the years and battles progressed, this position was mutating, and in 1863 President Lincoln made his Emancipation Proclamation, by which he freed all the African-American slaves that were in the southern states that were falling into the hands of the Union, urging in turn that they join the northern cause.
Thus, through these types of policies, President Lincoln was including slaves and abolitionists within his political position, leaving the Confederation in ideological check.
