Once Upon a Time in America

I just finished watching Sergio Leone's last film, Once Upon a Time in America. I'm not sure what to think. I found it very confusing. I'm not even sure of the basic points of the plot. It's a long movie (almost 4 hours) and I watched it over a period of several days, so the beginning parts are now too hazy in my memory to properly correlate with the ending. I checked out the message board on IMDB, which I don't usually do unless I really don't understand something (because people tend to start arguing and say really stupid and/or offensive things on them), and there are all these posts about the "Opium Theory." I should point out that at the beginning of the movie the main character is drugged out on opium and that the story is not told in a linear fashion, but rather with both flash-forwards and flashbacks. The "Opium Theory" holds that the flash-forwards are hallucinations made up by the character. I don't buy it. It is a very strange future, but not one that a character from the 30s would really be able to make up. I don't know. But I still don't understand who betrayed whom and why. In addition, while the violence in typical Spaghetti Western matches the over-the-top style of the movies, the violence in this was more intense and at the same time more personal. There is a particularly difficult-to-watch rape scene, as well. Anyway, if anyone has seen this movie, (if you haven't I haven't really given anything away), I'd be interested in their thoughts.