I do believe the southern white people realized what a dilemma they were in with a black African slave population that was by natural reproduction increasing its numbers at an alarming rate . There was no market for slavery in Mexico and the stopping of the spread of slavery in the U.S. would have effectively cut off the market for the surplus and left the already vulnerable white people of the south in the position of the white Haitian planters only 2 generations prior. The whites rightly feared such an uprising would be fomented, armed, and supported by northerners.
I am sure that by 1860 even the southerners realized what a huge mistake the importation of African blacks was. By then there was no sending them back to their place of origin. There were too many and all too often they were related by blood to the white folks. I know the 1836 inventory of my 3X great grandfather's estate described every one of the slaves as medium or yellow complexion. To sort of quote Shakespeare with an adaption "black slavery was a grievous sin, and grievously we have answered it". And we will continue to answer it from now on.
