Although most eventually assimilated, the process weakened the homogeneity and bonding of America. As the peripherally European, tired and hungry masses came in over the next half of a century, they encouraged America to turn its eyes back to the old world conflicts, and helped America become embroiled in the two great wars of the twentieth century. Such set the pattern of drawing us into the conflicts of the rest of the world, especially the Third World. As America went into those nations, so those nations are now in us. The conquered absorbs the conqueror.
Slavery is the great tragedy of America, and it is still the source of our greatest problems.
All this has nothing to do with its morality as an institution. Unjust institutions can be reformed or abolished, and a nation can survive, even prosper afterward. Its real damage has been that it placed a people in our midst who as a group have little aptitude for our technology, no empathy with our culture, no adulation for our history and heroes, and no love for us, only resentment for perceived past wrongs. We notice the racial attitude gap most readily on racial litmus tests, such as in reactions to the O.J. Simpson verdict, or the extirpation of George Washington from our schools. The stark reality is that the rift cannot be healed.