The truth is that the United States did not become what we would now recognize as anything approaching a liberal democracy until at least the 1920s, when Gitlow v. New York (1925) applied the Bill of Rights to the states by way of the Fourteenth Amendment. And getting the Fourteenth Amendment ratified necessitated the Thirteenth Amendment, ending chattel slavery. And getting the Thirteenth Amendment ratified necessitated, of course, the bloodiest war in U.S. history.
It's easy to portray the Thirteenth Amendment as the final victory of the abolitionist movement over the morally and politically putrid miasma that would later congeal to form the Ku Klux Klan, but the truth is that a decisive number of the opponents of abolition were always perceived as Southern moderates. The situation hadn't changed much by the 1960s.
Martin Luther King Jr. hinted at this in his letter from Birmingham Jail--which identified the movement's greatest "stumbling block" as "the white moderate, who is more devoted to 'order' than to justice"--and turning back the clock a century, we find the same reality in the milquetoast abolitionism of the American center. It took a war to shape things up in such a way that moderates were no longer able to keep slavery in place. That had a great many unexpected side effects, and one of those side effects was that it gave us, by way of the Fourteenth Amendment, a potent Constitution that can actually protect individual civil liberties in a meaningful way.