Love your logline, I can sense the story's strength!

I can also 'sense' the story's strength, but I do not find it in the logline. That the story is true does not automatically make it interesting --- as many people think, sometimes including producers. So I immediately cut that. Then we have:

In the civil war, a nun confronts General Sherman, the Union Army, and her convent's runaway slaves [about what?! (the loglines construction does not make a clear connection between [a] and [b] ] while trying to save 200 schoolgirls from the destructive burning of Columbia, South Carolina.

Does the European audience have any idea of General Sherman's involvement in the burning of Columbia the following article says that experts agree that he probably knew about what was going to happen/happened, but he did not order it?!

.... but Ok, that aside, it is after a book.

Maybe (without having read the book):

After General Sherman breaks his promise to protect Mother Superior's convent while taking Columbia, South Carolina, she must guide 200 schoolgirls to safety through a -blazing- hostile city.

Nuns standing up to generals - sounds good.