This is the story of the passing of the cowboys from the old West. Modernity killed them.
An honorable cowboy, apparently the last one of his kind, gets himself thrown in jail in order to visit (and spring out) his brother-in-law who has been unjustly imprisoned for aiding immigrants. But his brother-in-law refuses to live as a fugitive in the modern world, commenting that the old West which the cowboy romanticizes is long gone. So, the cowboy escapes alone.
Most of the movie is the cowboy trying to evade the policy and their modern methods of capture (including a helicopter) with only his wits an his horse. The cowboy immediately makes for México by cutting across a mountain --- his turf. In fact, the cowboy makes it, despite having the odds against him. But in the final sprint across a very modern highway, his horse gets spooked, and the cowboy, horse and all, gets run over by a very modern truck that is hastily on its way to deliver very modern toilets.
Modernity killed the cowboy. Literally.