I watched the original 2 movies (not the short 2,5hr western version of both movies) and although a brilliant set of movies I think if you want to understand/know the story about the three kingdoms you should read the whole book (FLP version, again not a shortened version) and not just watch this movie (be it the short or original 2 part movie).
The movies just cover a section of the book, the events leading up to the battle at Red Cliff are not explained, many events during the battle are also left out, and all the events after the battle of the Red Cliff are not in the movie(s) either...
Of course the book is a romanticized version of the true historical events, but that's another discussion again
