The third film (1971) in a trilogy directed by Howard Hawks, varying the idea of a sheriff defending his office against belligerent outlaw elements in the town: the other two films were Rio Bravo (1959) and El Dorado (1966), both also starring John Wayne (Wikipdedia). Hawks' last film of 47 (sic), among which Sergeant York, To Have and Have Not, The Big Sleep, Red River, and Gentlemen Prefer Blondes.
Rio Lobo is a praiseworthy last work of a great director; it is also better than its predecessors in the trilogy. Starts in the last days of the American Civil War (1861-65). Wayne, still quite youthful and far away from his last films with an oxygen tent nearby, starts as a Cavalry Colonel, who has a Union payroll train under his command. Despite all precautions, he train falls to the Confederates - clearly by a Union traitor in collaboration with some Confederate officers.
Well, Wayne is angry and morally annoyed, and swears to catch the gangsters: This is a case of treason, not war! His tracing leads him to the town of Rio Lobo, where, after many complications and the help of a number of younger women living in the area on their own - this is late horse only time, as railways have moved in, and also a late men only West - he achieves his target. The emancipation element, by the way, thanks to Hawks' discipline, is treated as a matter of fact (and is hence more impressive, though some female acting is a bit bouncy), and we are also spared sloganeering on the subject.
Acting, camera &c all ok. Five stars, even today, forty years later.