Question

What was Abraham Lincoln's reasoning when he refused to let the southern states secede?

Answer 1

For Lincoln, allowing American democracy to succeed was compatible with the ideal of freedom; allowing secessionists to destroy it (in response to a democratic election) was not.  In other words, Lincoln did not believe that true freedom was letting states do their own thing--and letting the pillars of American constitutional democracy run amok--but instead, in maintaining a union where the great experiment of democracy could flourish.  As Lincoln himself said quite clearly in the Gettysburg Address, he was  committed to making sure "...that government of the people, by the  people, for the people, shall not perish from the earth."  I suppose you can argue that Lincoln's vision of freedom was not worth the price, but you cannot deny that he had a vision of freedom--and that, for him, this vision was compatible with maintaining the historic, unprecedented political freedom that was achieved in 1776.