According to the southern states it was the abolitionists and the Republican Party to begin with. Then during the course of the civil war it became a war aim of the US to free the slaves.
Though to be honest it was the slaves themselves that started it. As soon as Union armies came close to slavery areas, slaves would run away and ask for protection from the Union Army. Kinda makes a lie out of the slavocracy’s claim that all the slaves were just happy as can be picking cotton.
“Then during the course of the civil war it became a war aim of the US to free the slaves.”
That is an interesting comment: “to free the slaves.” I heard it so many times when I watched television.
Followup question: was this U.S. war aim to free the slaves in the Union slave states, the Confederate states, or all the states?
Full disclosure: this is a trick question. You might want to skip it altogether and let Brother Joe get you out of the jam with some gobbledygook from the First Church of the Perpetual Union.
How did they get around that constitutional requirement to return them to their masters? Did they just declare that portion of the Constitution null and void?