Plainly, the central idea of secession, is the essence of anarchy. [Quotations of Abraham Lincoln. [Applewood Books, Bedford MA 2003: 11.]
If the central idea of secession be the essence of anarchy, the central idea of reunification by force is oppression. I know of no moral justification for forcing a minority to submit to a majority from which it wishes to separate itself. Such practice is the essence of ‘might makes right’ and thus precisely what ‘liberty and justice for all’ most strongly opposes.
Lincoln of course spoke in the context of violent secession, which constitutes a rather different practical matter and occurs in a different moral context. To say that a minority has the right to walk away from a group–as Americans did from England, after all–is a principle. If the majority refuses such a request, that violates the principle; if a minority chooses violence rather than first attempting negotiation, that also violates the principle.
As for the particular case involving Lincoln, we will never know if a national referendum would have awarded the South a peaceful exit, but a powerful argument can be made that the U.S. would have been a far greater country if a peaceful parting of ways with the racist and elitist Slavocracy had been achieved. Its hostility to the “American values” expressed in the Declaration of Independence were sadly evident still during the struggles for racial equality in the 1960s and again in today’s American brutality against Muslim societies. Since the South began the violence, the North could hardly have been expected, on either moral or practical grounds, to have submitted. To the degree that the Civil War was a war of industrial modernism against a landed aristocracy, it was aggression and thus wrong; to the extent that it was a war for American values, it may have been a justifiable war but sadly remains far from won.