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.
"a black African slave population that was by natural reproduction increasing its numbers at an alarming rate"
"I know the 1836 inventory of my 3X great grandfather's estate described every one of the slaves as medium or yellow complexion"
You do realize that a significant portions of that "natural reproduction" of the slave population was slave owners like your grandfather and his kin r
ping the slaves. There was also deliberate breeding with slaves with other slaves. Slaves where an asset that could be grown only through there reproduction, the slave women where expected to increase the owners wealth by having children.
"There was no market for slavery in Mexico.."
The South had its own version of Manifest Destiny. The South was as into expansion as much as the North was. There was a movement in the South to start a conquest of Mexico, Central America and the lands of Caribbean (The Golden Circle). Lincoln had only put forth a policy of stopping the expansion of slavery, I don't think people realize how seriously the South took that within the context of the mind set of the time.
