This week, 3:10 to Yuma hits theaters, telling the story of a cash-strapped rancher (Christian Bale) who volunteers to help escort a train robber (Russell Crowe) to federal court; what follows is a battle of wits in which the normal bounds of good and evil fall by the wayside. Thus, Yuma joins a long line of Westerns that explore the complexities of human nature against the unforgiving backdrop of the frontier.

In the mid-1960s, filmmakers began to move away from the black-and-white morality of earlier Westerns in order to explore some of the darker aspects of the frontier. That's not to say that earlier films didn't delve into the complexities inherent in the form; High Noon (95 percent on the Tomatometer), from 1952, presented a skeptical view of the nature of heroism, and John Ford's 1964 drama Cheyenne Autumn (63 percent) took a more sympathetic view of the plight of Native Americans than did his previous works. But it wasn't until later in the decade that Westerns began to depict the frontier in a more complex, ominous light: as a place of dirty living conditions, changing political and economic climates, and the razor-thin line between lawlessness and order.

One of the primary forces in the shift from the traditional Western was Sergio Leone, who created the template for what would become known as the Spaghetti Western. His revisionist oaters, shot in the deserts of Spain, featured iconic, antiheroic gunslingers and almost comic violence that shook up the staid conventions of Hollywood Westerns. Starring Clint Eastwood, Leone's "Man With No Name" trilogy (A Fistful of Dollars [94 percent], For a Few Dollars More [91 percent] and The Good, the Bad and the Ugly [100 percent]), as well as Once Upon a Time in the West (97 percent) with Charles Bronson are all classics of the genre. Lesser-known but also startlingly memorable is Duck, You Sucker (aka A Fistful of Dynamite, 81 percent), an epic meditation on revolution, class warfare, and explosives. Set during the Mexican Revolution, it tells the story of Juan (Rod Steiger) is a bandit who comes across Sean (James Coburn), an explosives expert and Irish revolutionary. After some setbacks, Juan and Sean team up to raid a bank, and Juan becomes a reluctant hero of the revolution when it turns out the bank is filled not with money but with political prisoners.

Duck, You Sucker remains one of Leone's least-seen films, but it's got a number of the same elements that make Leone's westerns so memorable: breathtaking vistas, explosive battle sequences, a haunting Ennio Morricone score, and oodles of moral ambiguity. Coburn doesn't match Eastwood's iconic, tight-lipped gunslinger, but he makes up for it with crotchety humor and an air of morality; he's committed to a cause, but disillusioned with the revolutionary life. And Steiger's Juan evolves from a selfish thief to a man who realizes his unique place in the midst of historical events. "The combination of Leone's obsessive close-ups, Ennio Morricone's melodious music, and the comradely chemistry of Coburn and Steiger ignite an emotional explosion comparable to that of Once Upon a Time in the West," wrote Andrew Sarris in the New York Observer.

As the west was tamed, the tolerance for gungsliners and bandits -- once lauded as antiheroes -- evaporated. Sam Peckinpah's The Wild Bunch is about a tight group of such undesirables, who realize their time is running short, and decide to go out guns-blazing. The aging outlaws that comprise the Bunch -- which include Pike (William Holden) and Dutch (Ernest Borgnine) -- mastermind a series of robberies in the dusty outreaches of rural Texas; their philosophy is encapsulated in the film's most famous line: "If they move, kill 'em." In between heists, the outlaws do their share of carousing, but there's a sadness underlying even the movie's most carefree scenes, a sense that these men share a bond, but that their specific brand of loyalty is quickly becoming outdated. Hot on their trail is Thornton (Robert Ryan), a former member of the gang who now leads a group of mercenaries to capture the Bunch, and is deeply conflicted about his new role.

However, don't be fooled into thinking The Wild Bunch is in any way a slog through the last days of the west. The movie has many moments of melancholy, but it also contains at least three battle scenes with a level of ultra-violence -- and sheer awesomeness -- that few have come close to matching (although many directors have tried; John Woo and Martin Scorsese have both cited The Wild Bunch as an influence. "The on-screen carnage established a new level in American movies, but few of the films that followed in its wake could duplicate Peckinpah's depth of feeling," wrote Dave Kehr of the Chicago Reader.

The end of the old west is one of the prominent undercurrents in Robert Altman's masterful McCabe & Mrs. Miller (91 percent). McCabe (Warren Beatty) arrives in the grim Northwest town of Presbyterian Church with the intention of establishing a saloon and a whorehouse. It isn't until the British madam Mrs. Miller (Julie Christie) shows up that his plan starts to take shape: She supervises the construction of the building and oversees the girls. The pair forms an uneasy-but-affectionate personal and professional partnership that contributes to the growth of the town. But powerful mining interests get wind of the business' success, and offer to buy McCabe out; drunk with success, he insults them, with catastrophic results.

Like the HBO series Deadwood (which owes an obvious debt to McCabe), one of the underlying themes of the film is how the freewheeling, pioneering spirit of the frontier was tamed by economic -- and often uncivil -- means. But that only goes so far in describing the poetry of this masterful film. You'd be hard-pressed to find a movie more evocatively, forbiddingly atmospheric than McCabe & Mrs. Miller. The harsh environs of the snowy Northwest are so vivid you can practically feel a chill from the screen. In addition, the sepia-like cinematography of Vilmos Zsigmond and the elegiac songs of Leonard Cohen create a forlorn, bleak feeling of a time and place that will soon be relegated to history. "Robert Altman has made a dozen films that can be called great in one way or another," wrote Roger Ebert. "But one of them is perfect, and that one is McCabe & Mrs. Miller."

The Western has gone through a number of cycles, and though it's fallen out of favor in more recent times, there are always a few oaters, like Unforgiven (96 percent), Open Range (78 percent), or The Proposition (86 percent) that explore the landscape of human complexity -- usually with a shotgun in hand.