“At the time of the war, the South (i.e. the Confederacy) had repudiated the entire Constitution of the United States . . .”
Actually, the Confederate States adopted the United States Constitution almost in its entirety. The Confederates made a few improvements and clarifications based on experience but it was almost the same words that southerners had written and offered to the nation two generations earlier.
Even after secession, the United States remained the most powerful slave nation in North America, perhaps the world.
None the less, it was the Confederacy that repudiated the Constitution of the United States and withdrew from the Republic.
Yes, the United States was the most powerful slave holding nation in North America with about 430,000 slaves. The Confederacy held the largest number of slaves in North America about 3.5 million slaves in all.