A noir tale folded into a south of the border western - at the leading edge of the wave of morally ambiguous Westerns. Burt Lancaster produced the film and plays the less honorable (much less honorable) of the two leads. Cooper isn't always at his best here, but effective anyway. Lancaster is a treat in his villainous role.
Smoothly directed by Aldrich, and with a terrific cast (Cooper, Lancaster, Cesar Romero, Ernest Borgnine, etc) this deceptively bleak film's an effective bridge between traditionally heroic westerns and the more amoral frontier films of the late 50's and 60's. Nobody trusts anybody else in the film really, but it's having a soft spot that keeps a gunslinger alive---belief is its own reward, given the alternative.
To me the ending defines the principles on which auteurism is based; a final shot robbed of the triumph of romantic reunion, and instead a man walking into the distance after committing terrible, inevitable violence.