One of the things that makes New York City unique and remarkable is its sense of social experiment. No other city in the world mixes so many different cultures and nationalities within one very crowded island, and that combustible recipe has never seemed more successful than in the unified spirit that healed the city after the tragedy of September 11, 2001.

It wasn't always that way, as Martin Scorsese's monumental Gangs of New York so boldly and bluntly illustrates. Scorsese's longtime dream project brings to the screen a little-known chapter of U.S. history--the fierce rivalries between Irish immigrant gangs and anti-immigrant "Nativists" in the slums of Lower Manhattan in the mid-19th century. The result is something startling--a New York western, with all the visceral power and meticulous craft one has come to expect from this great American filmmaker.

Loosely based on the 1928 book by Herbert Asbury, Gangs centers on the struggle of Amsterdam Vallon (Leonardo DiCaprio), who, as a child, watches his Irish gang leader father Priest (Liam Neeson) die at the hands of the coldblooded Nativist William Cutting (Daniel Day-Lewis), aka "Bill the Butcher." Years later, after being shipped off to reform school, Amsterdam returns to his tough Five Points neighborhood, hungering for revenge. The megalomaniacal Butcher dominates the area, in league with Tammany Hall's corrupt Boss Tweed (Jim Broadbent). Keeping his identity a secret, Amsterdam becomes a protg of Bill's, and is troubled to discover that when you're taken "under the wing of a dragon, it's warmer than you think." The conflicted Amsterdam even goes so far as to protect Bill from a would-be assassin, but soon his cover is blown by his best friend Johnny (Henry Thomas), who has become his rival over a comely pickpocket named Jenny (Cameron Diaz).

The script by the first-rate trio of Jay Cocks, Steven Zaillian and Kenneth Lonergan isn't strong on narrative--the blood feuds and jealousies are the stuff of 19th-century melodrama--but its value is in the details. The historical forces at work here are vividly dramatized, and the writing brims with wonderfully colloquial dialogue, lines like "She's a prim-looking stargazer" and "I come for my due and proper." Perhaps no surprise, the movie's most entertaining character is also its most despicable: Bill the Butcher. Played with a thick upswept mustache, a self-satisfied grin and a nascent New York accent by a wickedly scene-stealing Day-Lewis, Bill is one of Scorsese's most memorable villains, taking delight in his own flowery declarations as he sadistically carves his way through the Five Points. No wonder Amsterdam feels so ambivalent.

DiCaprio, with long hair, wispy moustache and beard, and a little extra poundage, leaves behind most of the boyishness he still displays in the concurrent Catch Me If You Can--he's been a solid actor from the beginning, and he ably handles the emotional dilemmas of his character and the transition to a more muscular adult role. Diaz is perfect casting as a feisty vixen, and also shows growth in her later dramatic moments. Neeson gets the movie off to a powerful start as Amsterdam's doomed father, while Broadbent, Thomas, John C. Reilly and Brendan Gleeson add luster to the supporting cast.

As important as any of the stars are Dante Ferretti's incredible sets, built by the artisans at the legendary Cinecittà studio in Rome. The massive Five Points neighborhood set is one of the most wondrous in years, several blocks long and painstakingly crafted in fully persuasive period detail, dominated by the rickety Old Brewery which houses new arrivals. Even the New York Harbor is recreated, scene of a virtuosic tracking shot in which newly arrived immigrants are signed up and shipped out as soldiers in the Civil War while dead bodies arrive home in rows of wooden coffins. The war plays a huge, almost competing, role in the movie's climax, as immigrants reject the government's draft program and riots break out all over the city--a tour de force of Scorsese mayhem and a showcase for more spectacular Ferretti sets.

Though some viewers will be put off by the stylized violence and generally downbeat content, Gangs of New York is a mightily impressive recreation of a fascinating piece of history. This long-gestating project could bring a long-overdue Oscar to one of America's most invaluable filmmakers.

-Kevin Lally