A while back, I posted a short essay about "Gangs of New York" (2002), discussing its portrayal of a party system de-polarizing and then re-polarizing around the issue of immigration. Jon Bernstein has a nice follow up touching on the film's treatment of interest-based politics.
One other feature I wanted to mention was the film's examination of the rule of law. A recurrent theme in the film is the utter lack of a central agreed-upon authority. Decisions are made by gangs and party organizations. If you're not a member of the gang or party that made the decision, you regard the decision as illegitimate, and the main recourse you have is killing the people in charge and making your own decisions.
A great example is an early scene when a fire breaks out in an apartment building. A fire brigade rushes to the scene, but they are not the fire brigade. Another fire brigade, sponsored by another gang, soon shows up, and rather than fight the fire, the two brigades fight each other. It's funny, but also quite telling about the costs of not having any sort of central acknowledged government. (It was also kind of ballsy for Scorsese to portray firefighters as thugs a year after 9/11.)
The lawlessness theme in the film is best exemplified in a conversation that Bill "the Butcher" Cutting (the local gang leader portrayed by Daniel Day Lewis) has with the area's top police officer, Happy Jack (John C. Reilly). Bill is instructing Jack to go hunt down and kill Bill's young rival, Amsterdam Vallon (Leonardo DiCaprio), but Jack appears reticent. As Jack says, "My allegiance is to the law. I'm paid to uphold the law." Bill thinks for a bit, and finally says, "What in Heaven's name are you talking about? You may have misgivings, but don't go believing that, Jack. That way lies damnation."
The line comes off somewhat humorous, but there's a real point to it. Bill has literally no idea what Jack is talking about. The notion that there is some legal principle of impartiality to which a government official is bound is completely alien to him.
The film's finale brings an end to this form of lawlessness. The old code of rival gangs ruling through force and some ancient code of honor ultimately gives way to the rule of the federal government. Of course, it does so through force: the U.S. Navy bombards lower Manhattan during the 1863 draft riots, vanquishing the gangs and clearing the way for Army soldiers. It's an open question whether rule of gangs has been replaced by rule of law, or whether the federal government was just the biggest gang in town.