After January 1 1863, Every slave in those states in rebellion against the United States became de jura free. Where ever the Union Army went after that date, slaves became not only de jura but de facto free persons. By the end of the war, the Union Army freed about 3 million slaves.
Does that give you an idea of who was fighting to end slavery?
And after that, President Lincoln added yet another slave state to the most powerful slave nation in North America - the United States.
Brings to mind the observation of the London Spectator: “The Government liberates the enemy’s slaves as it would the enemy’s cattle, simply to weaken them in the coming conflict . . . the principle asserted is not that a human being cannot justly own another, but that he cannot own him unless he is loyal to the United States.”