And thanks for mentioning that bio. I know his autobio has been widely read, haven't got around to reading it yet.
There's a great quote on his Wikipedia page:
Boyd later recalled witnessing that, when news of South Carolina's secession from the United States reached them at the Seminary, "Sherman burst out crying, and began, in his nervous way, pacing the floor and deprecating the step which he feared might bring destruction on the whole country."[63] In what some authors have seen as an accurate prophecy of the conflict that would engulf the United States during the next four years,[64][65] Boyd recalled Sherman declaring:
You people of the South don't know what you are doing. This country will be drenched in blood, and God only knows how it will end. It is all folly, madness, a crime against civilization! You people speak so lightly of war; you don't know what you're talking about. War is a terrible thing! You mistake, too, the people of the North. They are a peaceable people but an earnest people, and they will fight, too. They are not going to let this country be destroyed without a mighty effort to save it ... Besides, where are your men and appliances of war to contend against them? The North can make a steam engine, locomotive, or railway car; hardly a yard of cloth or pair of shoes can you make. You are rushing into war with one of the most powerful, ingeniously mechanical, and determined people on Earthright at your doors. You are bound to fail. Only in your spirit and determination are you prepared for war. In all else you are totally unprepared, with a bad cause to start with. At first you will make headway, but as your limited resources begin to fail, shut out from the markets of Europe as you will be, your cause will begin to wane. If your people will but stop and think, they must see in the end that you will surely fail[66].
The phrase "Boyd recalled" gives me a little pause; perhaps there was some editing, or creative memory? I'm willing to trust that it captures the essence of what was said, though maybe not in exactly those words.