"The material destruction of the war was not so great but that it could by this time have been repaired, had a good peace been made without delay." I changed "war not so great" to "war was not so great". I'm guessing that omission was a typing error by you ?? The structure you mention sounds very awkward to me. I understand it, but the whole sentence is unnecessarily complex and difficult to understand on first reading. Others can tell you if it is grammatically correct, but I wouldn't word it like that ! My rewording would be more like: "The material destruction of the war was not so great that it couldn't have been repaired, had a good peace been made without delay." It'll be interesting to see what others say.