The South gave the North two options after the Civil War: draconian rule by the North enforced by hundreds of thousands of soldiers that would be resisted by guerrilla warfare, or they could allow the South to re-join the Union on an equal basis as long as they pretended to no longer have slavery and pretended to be part of the United States. Given that one option meant generations of Union soldiers in the South and the other option meant peace, it’s no wonder that in the compromise of 1876, the North accepted the South’s offer of the second option.
Still, during the ten years that the North attempted to enforce democracy in the South, they should have taken the opportunity to hang the whole lot of traitors who were the top military and civilian leadership of the Confederacy by the neck for treason. Instead they let those people go free and create the legend of the “Lost Cause” where it wasn’t about slavery, it was about a way of life, and the North won not because they had better generals and better political leadership (which, at the end of the war, was 100% true — Lincoln had weeded out the weak leadership ruthlessly and arrived at a team that was ludicrously effective), but because of sheer industrial might.
The reality is that the South had better guns and powder for most of the war, and if everybody who had been drafted had shown up to fight, would have been able to match the North man-for-man. The South lost because of failures of its political and military leadership, not because of lack of manpower or lack of weapons — but by allowing the traitors to live, the North allowed the traitors to re-write history to where they were the valiant victims of Northern industrial might, rather than incompetents who lost the war through their own stupidity and arrogance. By allowing the traitors to live, the North gave credence to their arguments that treason was an ordinary and respectable thing. Thus the North, eventually, lost the Civil War, if not in 1876, then most certainly when a tool in the pay of the Russians became President with Russian help because treason — accepting the aid of a foreign power in expectation of quid pro quo — has been rendered an ordinary and respectable thing.
— Badtux the Geopolitical History Penguin