It's taken many years for me to come to this conclusion, but I have, finally, reached the libertarian extreme of regarding Lincoln as a tyrant; the Civil War emplaced a "Hotel California" policy on the individual states of the union: you can never leave. It's impossible to know how long the secession would have lasted without violence if the Confederates had not fired on Fort Sumter, but it was USA policy from the get-go that secession was out of the question.
The various territories and principalities which were under US control following WW2 were wise--however inadvertently--to vote for independence, rather than being an American fiefdom like Puerto Rico, because joining the USA is a one-way ticket.
It's unfortunate that the founding fathers didn't foresee any circumstances where a state might want to leave the union, else they might have written a procedure for doing so. (Though my command of colonial history is insufficient for me to say, one way or another, that one wasn't written, and subsequently excluded from the Constitution.) It is certainly true that many states would not have joined in the first place if they'd known they'd be kept from leaving under threat of war. Having such a passage in the Constitution would have mitigated the accumulation of power in the federal government, and it would have left Lincoln with no constitutional alternative when the Confederacy seceded. That's not to say that war would not still have resulted, but it'd be a little more obvious who was right and who was wrong.
Banning the Confederate flag is the kind of blinkered nonsense we used to scoff at in other countries, like German schools not teaching much about Nazis, or Japanese schools not teaching about the Rape of Nanking. We used to scoff at that because Americans never flinched from looking at their own history--we stood up and said, for example, "Yeah, we dropped those two atom bombs."
But like Germany and Japan assiduously not talking about their embarassing contretemps, the Democrat Party wants not to be reminded of its own. Democrats want everyone just to forget that the Democrat Party was the party of slavery and Jim Crow and institutionalized racism and human trafficking and the Confederacy. That is what this is really about.