I am told by those who saw the movie "Gangs of New York" that Ken Masugi nails it. It’s too bad, Scorsese blew it. Masugi’s first paragraph gives you the crux:

Movies about gangs often raise enduring questions about justice and becoming American. Consider "The Godfather," which begins with the line, "I believe in America"; "The Warriors," which used a multiracial gang to retell Xenophon’s Anabasis; and "West Side Story," with Rita Moreno singing "America." For all its epic strivings, "Gangs of New York" never achieves these heights, for it stubbornly maintains the perspective of tribes.