Emancipation Proclamation--this proclamation stated slaves were free in Confederate areas but to proclaim their freedom, they needed to get North.
The effort of slaves to escape or turn on their masters during the Civil War made the war about liberation for them. They were fighting for freedom instead of a political system or law like the Union and the Confederacy were. In the case of the 54th Regiment, the only all black regiment, blacks were given an opportunity to fight for their freedom.