As an undergraduate, I majored in history. I'm also a big fan of the science fiction subgenre, "alternate history." Therefore, I very much enjoyed the book What If and its sequel What If? 2, both filled with essays written by historians who ask that question about several pivotal points in history. What if things had gone the other way, what if the losers had somehow won? How would history of gone then?
CNN has a review of the first book, "Book asks what might have been"