Early in Once Upon a Time in America, Noodles (Robert De Niro, playing this character as a man) remembers how he used to stand on a toilet and peek through a crack to watch Deborah (Jennifer Connely, playing her as a teenager) dancing in the storeroom where there were sacks of flour. The dust from the flour creates a haze, giving the scene a dreamlike quality. She dances to “Amapola,” a song comparing a pretty girl to a poppy, the flower from which opium is derived. Together these elements form a constellation of themes running through this movie: Deborah/dream/love/opium.
Deborah knows that Noodles (Scott Tiler, playing him as a teenager) is watching her, and she surprises him by opening the door, calling him a roach, and saying, “That record’s just like Ex-Lax. Every time I put it on, you have to go to the bathroom.” It is to be noted that she does not enter the restroom. Later in the movie, Noodles goes into another restroom and sits down on the toilet, unlocking the door when he realizes Peggy is coming, so that he can expose himself to her. Unlike Deborah, Peggy walks right in, and Noodles spreads his legs. He gets up from the toilet and starts making sexual advances. She says she is about poop in her pants, after which she plops down on the toilet he just got up from, telling him to get out. In fact, throughout the movie there are innumerable references to garbage, excrement, and anal sex. Taken together, all this leads to an opposing constellation of themes: Noodles/reality/sex/filth.
As the movie jumps back and forth in time, it always seems to heading toward the end of Prohibition. When Noodles and Max (Rusty Jacobs, playing him as a teenager) first become friends, Prohibition has only recently become the law of the land. Their friendship comes to an end just before the repeal of Prohibition becomes effective, when Noodles believes he has caused the death of Max (James Woods, playing him as a man).
Furthermore, Noodles’ love for Deborah begins around the same time, in the early days of Prohibition, and his dream of marrying her comes to an end when he brutally rapes her (Elizabeth McGovern, playing her as a woman). The next day, he sees her depart on a train. He looks at a newspaper, the headline of which announces the end of Prohibition. This is regarded by some critics as a goof, because in subsequent scenes, reference is made to the fact that Prohibition has not ended yet, though it soon will.
However, a scene occurring many years later, in 1968, makes it clear that this connection to the end of Prohibition is not intended to be understood realistically. Noodles finds out that Max is still alive. As he is leaving Max’s mansion, there is the mysterious scene with the garbage truck, followed by what appears to be a bunch of revelers in a car made in the 1920s, as we hear the song “God Bless America.” The sounds they make and the background music being played are identical to what we heard at the beginning of the movie, as Eve prepared to enter the apartment she shares with Noodles. They are the sounds of people celebrating the end of Prohibition. In other words, even in 1968, it still seems to be the end of Prohibition. That is because what happens in 1968 is actually just a dream, a dream that is taking place at the end of Prohibition. One problem with this theory, however, is that Noodles would not know about 1968 technology, like television. But that can be justified as dramatic license.
In any event, Prohibition is the period during which Max and Noodles were great friends, and when Noodles’ love for Deborah was still something pure and beautiful. The end of Prohibition is the end of both his friendship with Max and his hopes of marrying Deborah, and that is why it takes on such significance in this movie.
After Noodles rapes Deborah, he tries to forget what he did by spending time in an opium den. This is referred to, but not seen. We do see two scenes of him in the opium den, however, both of them being after he thinks he has killed Max. The first time is at the beginning of the movie, where we see him leaving; the second time is at the end of the movie, where we see him entering. Bookending the movie in this way, with the leaving being seen in the beginning and the arriving at the end, we are encouraged to see the movie as Noodles’ opium dream. As his dream takes place at the end of Prohibition, the scenes set in the future are not real, but only part of his wish-fulfilling dream. In that dream, he denies the reality of Max’s death, and imagines that it was really Max who betrayed him. Furthermore, by having Deborah be Max’s lover, and the mother of Max’s child, his dream makes it appear that she betrayed Noodles, in a sense, thereby alleviating his guilt over having raped her.
When Noodles raped Deborah, he dragged her out of the dream/love/opium constellation into his own world of reality/sex/filth, because he was angry that she was leaving him, instead of marrying him, as he had always hoped. In a similar way, when he convinces himself in his dream that it was really Max who betrayed him, he gets his revenge by dreaming that Max jumps into a garbage truck, where he is ground up and turned into garbage himself.
In the final scene, which is in the opium den, we see Noodles take a puff on the opium pipe. The expression that suddenly appears on his face is one of happiness, but it is only the false kind of happiness that opium provides, a temporary illusion that leaves a bitter taste in the mouth. By extension, life itself is just so much filth, and what little happiness we find is like that produced by opium.