Odd western with Big John as an aging Marshal whose sons go astray because daddy isn't home enough. They absolutely want his attention and therefore make some 'bad friends' and get involved in a bank robbery. As you might have expected, things go terribly wrong; nobody was supposed to get hurt, but somebody does, and instead of bad friends, the bank robbers turn out to be real mean bastards.
The premise of Cahill US Marshal seems more suited to a contemporary drama than a western, and indeed, it was intended to be a movie about a cop and a widower, who's more busy with his job than with his two growing up kids at home, but in the last minute, they decided to turn the whole thing into a western (Wayne did a couple of cop thrillers in those days and they weren't very successful).
Wayne looks a bit tired, but he saves himself with his usual bravura and turns in a pretty good performance. George Kennedy (as the leader of the boys' 'bad friends') and Neville Brand (as a halfbreed tracker) who assists Cahill on his quest, are also very good. The boys (especially Gary Grimes) are a bit irritating. Some good (quite violent) action scenes, some good location work across the border (Durango), but McLaglen's direction is uninspired and there are too many scenes shot on sound stages.