In the Declaration of Independence, it declares that "all men are created equal" but the founding fathers did not all believe that this applied to slaves. Many of them owned slaves. It took a Civil War to effect this change, but it did not happen right away. Although slavery was abolished and the Constitution amended, it took much longer to change the hearts of people. Slavery was outlawed, but Blacks were certainly not treated equally, especially in the Jim Crow South. It wasn't until the Civil Rights Movement of the 1960s that this was changed.
Although slavery was a key issue in the Civil War, it was not the only reason for the war and many historians believe that slavery was only one part of the real reasons for the war -- sectionalism. Therefore, I will give you a perspective that you might not have considered.
In the South today, some people still refer to the Civil War as "the war of northern aggression." The Declaration of Independence also states this:
That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, — That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness.
This founding principle is what led the southern states to believe that they could secede from the Union. The southern states were not “consenting” to what the government was doing, so therefore their needs were not being addressed (economic being a big one regarding taxes, tariffs, as well as the slavery issue which was at its core, an economic issue) and they could abolish the government and form their own. The southern states favored states’ rights over the rights of the federal government. The northern states felt just the opposite, favoring a “federalist” position (remember The Federalist Papers?). The South lost the war, so from their point of view, the founding principles were not successful.
I am playing the Devil’s Advocate here because I am not from the South, but this is a differing viewpoint. Perhaps some southern colleagues can chime in with opinions to give you some variety for your answer.
I think the founding principles of this nation can be seen in the Declaration of Independence. The Declaration states that “all men are created equal” and that they are endowed by their creator with “certain unalienable rights” and that among these rights are “life, liberty, and the pursuit of happiness”. Before the Civil War, these founding principles were not a reality in the U.S. because of the existence of slavery. One outcome of the Civil War was the passage of the Thirteenth Amendment which ended slavery in the U.S. This definitely was a step in realizing the founding principles of equality and liberty.