Slavery was the primary reason for the slave power to pretend to secession.
Union was the primary reason for the US to oppose secession.
At the end of the war, slavery was dead. The slave power lost. Anyone sad about that?
But the ramifications of that loss extended far beyond the end of slavery and we will see those issues again. Our Federal Government was supposed to be a Federal Government (very limited in power) while the State Governments were supposed to have more power. Instead, we now have a National Government, and the States have been stripped of much of their power. That is regrettable, as the National Government imposes one-size-fits-all standards where one size doesn't fit, while the states have become rubber stamps for the National Government.