Successful Economically
That slavery did not pay is generally a well-meaning myth. In the American South it profited so well that Southerners insisted on expanding slavery into other states. The issues surrounding that led up to the American Civil War. The mechanizing of hard labor helped lead to opprobrium of slavery.
From 1840-1860, the economic growth rate of the old Confederacy was similar to that of China for the past 20 years.
The benchmark study of the economics of American slavery is Robert William Fogel and Stanley L. Engerman, Time on the Cross, Little, Brown, Boston, 1974. Legislation like the Fugitive Slave Act compelled those opposed to slavery to abet its preservation, and such laws were enacted because the economic rewards for doing so were so great. Some surmise that had only one or two such laws failed to pass, slavery in the United States might have ended as peacefully as in Britain.
Slavery phased out in Britain during the century preceding the American Civil War, but Britain aided the Confederacy during that war. American cotton was a valuable resource for British textile mills. Foreign policy was as "pragmatic" then as now.