Now before anybody gets the wrong idea. I completely agree with Apostate that the ending of slavery in America was a good thing.
And I will concede that given the political situation at the time, probably the only way it was going to end was thru violence.
If cooler heads had prevailed then slavery would have died a natural but much slower death as it was already economically un-viable.
But when have those in power, when threatened with the loss of their power, displayed 'cool head'?
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willowfall
In any case it would have happened due to economic considerations.
Bottom line, an Indian road worker probably works harder in any given day than any 50 Australian road workers, but 1 Australian road worker driving a levelling/grading/asphalting/compressing road working machine for a day would probably create more road than any 500 Indian road workers in a month.
Technology obviates labour, and slavery (in the US and elsewhere) was primarily justified on the basis of the productivity of the individual slave and the costs of maintaining him or her.
In the 1860's the water/man/animal powered Cotton Gin was already obsolete and rendered uneconomic by the massively more productive and unit cost cheaper Northern States cotton mills, slaves were already being superseded by much more efficient, cheaper and more productive cotton picking machines, and the amount of money spent of housing, feeding, and otherwise caring for huddle expensive (at the time around about $1000 average per average ... not prime ... field slave - which was more than 95% of the white population earned in a year) slaves had pretty much rendered slavery something that would have died out on its own.
That said, I think that those who argue the Civil War was purely a States Rights conflict that became an Emancipation one are unreconstructed revisionists who fail to take the temperature of the times.
England and Europe had wrestled with a death with the slavery problem, but during the 1850's the US was noted for having a moral/ethical hot blooded somewhat murderous disagreement (especially on the North/South bordered states) that culminated in John Brown and some serious massacres and violence by both sides.
The North because it had industrialised and had made slavery economically irrelevant could afford to be moral and on their high horse, whilst the South, which had not, was looking at the destruction of a way of life that served only what we would call 'the 1%' today ...
It was unfortunate that the extremists on both sides carried the day and plunged the US into a disastrous Civil War that cost better than a 500,000 lives ... but on the other hand, the conflict was the final making of America as a nation.