New York in the 1860s was a powder keg of racial, ethnic, and class tensions exacerbated by huge influxes of Irish immigrants escaping the famine, who vied with recently emancipated slaves and other free blacks for low-paying jobs; by an entrenched system of political corruption that allowed the rulers of the city to operate blatantly above the law; and by a conscription plan President Lincoln put into action to replenish Union armies decimated by war in the South. In July 1863, the powder keg exploded in four days of riots, looting, lynchings, beatings, and arson. As many as a thousand people may have been killed... a far greater percentage of the city's population at the time, around 800,000, than the death toll of 9/11 was.

Betcha never knew that. Or maybe it was mentioned in passing in 8th-grade social studies.

Martin Scorsese reminds us with Gangs of New York -- a stunning re-creation not only of period places but also of the period mindset -- how ephemeral even the biggest disasters and the most profound events are in the grand scale of history, even over the relatively short term. As soon as the last person with memory of the time dies, the period is suddenly as remote as the Roman Empire or Egypt's Upper Kingdom. Movies extend our memory, though, especially one so meticulous in its depiction of a time and place lost to us.

The Five Points neighborhood of Manhattan doesn't really exist anymore, but the area -- just north of Foley Square, now the borough's courthouse district -- was, in the 19th century, a notorious slum teeming and vibrant with poor immigrants from all over Europe and China, as well as free blacks, and it was here that the Draft Riots were fermented. But Scorsese (Bringing Out the Dead, GoodFellas) and his screenwriters -- Jay Cocks, Steven Zaillian (Schindler's List), and Kenneth Lonergan (You Can Count on Me), working from a cult 1928 novel by Herbert Asbury -- don't tell us the story of the Draft Riots so much as they show us the atmosphere -- ruthless, unforgiving, mired in dire poverty -- that directly caused the Riots.

Gangs is a classic revenge story: Amsterdam (Leonardo DiCaprio: The Beach, Titanic) returns to Five Points from a long stint in an upstate reform school to avenge the murder of his father, Priest (Liam Neeson: Star Wars: Attack of the Clones, The Haunting), leader of a gang of Irish immigrants, at the hands of Bill the Butcher (Daniel Day-Lewis), leader of a gang of "Natives" (not Indians, merely the descendents of earlier immigrants), when Amsterdam was a small child. But it's the almost alien setting of the tale -- so magnificently brought to life -- that makes the film so tremendously powerful.

The Riots are but a backdrop in the finale of Amsterdam's vengeance, but in bursting forth simultaneously, we're sucked into the atmosphere of the city -- lawless, chaotic, explosive -- through an intense identification with Amsterdam. DiCaprio comes into his own as a mature actor here, fiercely passionate and maintaining a slow burn of rage through Scorsese's epic runtime. And Day-Lewis's terrifying performance as the psychotic Bill is a violent immersion into a city ruled by gangs, not only the Butcher's Natives and other Five Points mobs like the Plug Uglies and the Bowery Boys, but also the political gang of Tammany Hall and Boss Tweed (Jim Broadbent: Moulin Rouge, Bridget Jones's Diary). Even the makeshift police squads and fire brigades of the time were endlessly warring tribes, as Scorsese shows us with his trademark black humor.

The film so overflows with raging, grim fury that while the image of a recently deceased bunny mightn't normally inspire dread in us today, both the terror and the loyalty inspired by Priest's gang, the Dead Rabbits, is clear. When Amsterdam returns to resurrect the Rabbits, scattered and powerless since his father's death, you don't have to know that in the slang of the time, "dead" meant "best" and a "rabbit" was a hooligan to grasp the import of the name and their symbol. Likewise, while it might be difficult to imagine 50,000 New Yorkers violently rioting today, as they did in 1863, Gangs makes it not only perfectly understandable but perfectly reasonable that they would.

This is an immensely potent film, often disturbingly violent, neither of which is unexpected from Scorsese. What is surprising is how relevant he makes a forgotten historical tale, connecting it to us today through a simple final dissolve from Lower Manhattan afire in 1863 to the just-pre-9/11 skyline: This too shall pass, and be forgotten.