My Grandmother had a saying, "Sweep your own door step, before you start sweeping around the door steps of ours."
Slavery was (in the terms of present day arguments) the "Law of the Land". That means the Constitution of the UNITED States. The Confederacy did not have to go to war to protect "Slavery".
Check your history of the Ratification process of the 1787 Constitutional Convention to see why that provision was included and it wasn't because of the four southern States Virginia, North and South Carolina , or Georgia as you were told in your history classes told by northern teachers and history book writers. According to the ratification provisions those four states were not necessary for the Constitution to become law, IF the northern states ratified it. Check New York and New Jersey stance on that provision inclusion. Actually the largest known slave cemetery in the United States is called Manhatten Island.
Slavery in the UNITED States did not end until December 1865 with the passage of the 13th Amendment changing the "Slavery clause" in the Constitution. The question is wound that Amendment have passed if the 11 southern states been under a Federal Occupation Government at the time. It barely passed as it was because of Northern States voting against it.
The Emancipation Proclaimation did not free the several million slaves in the north or occupied southern states in January 1863. The word "Contriband" did not come into being until the Treaty between the United States and Great Briton to corporate in controlling "the Slave trade on the High Seas", Ratified by the U.S. Congress in APRIL 1863. The main United States political issue and purpose in this treaty was to keep England from supporting the Southern Government not eliminating slavery. This caused the South to invade Pennsylvania and Gettysburg.
There are many more examples which are left out of our "Popular" history. Slavery was a very profitable business for NORTHERN slave traders. The politics to protect Northern slave owners such as the 1862 Kentucky slave act which allowed northern sympathizers slave owners to keep teir slaves while southerner were forced to free theirs. And because of that law Abraham Lincoln was a SLAVER OWNER the day he died because his father-in-Law (Mary Todds father) was keeping the four slaves he gave Mary as a wedding present. Lincoln didn't care they to Washington DC because he would have been forced to free them under a 1850 Washington Law that outlawed Slaves in that city. During the course of the War some 3,000 slaves were freed (paid for by the Government) and shipped back to Africa.
U.S. Grant sold his slave in Missouri before the war.
So the North and its leadership was not so "Lilly White" in "Freeing the slaves". And there were more pressing issues that caused the South to seek its Independence. Such as what we have today a Federal Government that could not be trusted to obey the law, and enforce that law fairly.
I could go on but that is useless to those who do not want to hear.