This is one of the finest American motion pictures that's ever been made. Director Sam Peckinpah's masterpiece that set the tone for the future of the western genre but more importantly it's a film that deconstructs the legends and myths created by Hollywood of the history of the American frontier.
The themes of contradictory loyalties and the absence of any recognisable code for the protagonists goes against everything that filmmakers like John Ford or Howard Hawks presented (John Wayne is said to have hated this film). For a start the setting is unusual, this is set in 1913/14 on the Texas/Mexico border. A violent civil war is raging in Mexico as rebels are attempting to overthrow the corrupt and dictatorial government. A small gang of criminals led by Pike (William Holden) flee across the border after a robbery of a railroad office goes awry. They are being pursued by a motley bunch of mercenaries hired by the railroad to catch or kill them all. Desperate the gang agree to rob an arms train for a Mexican General but the decision to give some of the stolen guns to the rebels results in the bunch having to decide on what they want from their wasted lives.
The film has two very famous set pieces that bookend the film; an opening gun battle as the gang's robbery in Texas goes wrong and a climactic battle against overwhelming odds that finishes the film. This is often thought of as a gratuitously violent film but this is not so, indeed by today's standards it's quite tame, but it is one of the first films to use squibs to simulate gunshot wounds and became controversial as a consequence. But actually the film's depictions of death are balletic with Peckinpah using a unique style of cuts and slow motion to highlight the majestic nature of the action. It's been much copied since but never as good as here.
There's been whole books written just about this film which highlights the high regard in which it is held. It is a genuine film masterpiece, a film that has much more going on than a casual viewing reveals. It rarely gets any better than this.