If Stephens spoke that strongly in private about the dangers of civil war, it would have been nice if he'd done so in public. Did he?
I tend to distrust these accounts of dire warnings. Someone may have had qualms at the time, a sense that things were going wrong, and in retrospect build that uneasiness up into thundering prophecies.
Catton's footnote cites Life of Alexander H. Stephens, by R.M. Johnston and W.H. Browne. Where those authors got the quotes I can't say. I have seen Stephens's prophesy elsewhere. It doesn't seem like the kind of thing a politician would broadcast to his countrymen.
Catton cites Life of Alexander H. Stephens, by R.M. Johnston and W.H. Browne as his source for the Stephens quotes. Where they got it I can't say. That kind of prophesying wouldn't have been well received among his fellow southerners, I shouldn't think. As a politician he would have been reluctant to offend them with such unpopular words.