Well, a few with military training and remarkable foresight, like Sherman, see what's ahead. Mary Chestnut seems to understand with her general sense of dread and foreboding that she can't shake.
Don't forget that Chesnut put the diary together after the war. She used a ton of notes she had written on the dates given in the diary, but she made many revisions and added material to make it more commercially valuable. I don't hold it against here. She was trying to make a living during reconstruction. But if she seems a little too prescient or too knowledgeable about things that people of her class didn't normally learn about until later, then it might have been added later.