Many years ago I saw a movie about a small group of confederate soldiers that went west dressed in civilian clothing. I believe they were on a special mission of some sort. The movie ended with a group of other soldiers approaching and they looked like they were confederates, but when the approaching group stopped they shook the dust off their uniforms and they were blue under all the dust (Union soldiers). The confederate group tried to get away but ended up in a box canyon, they grouped in a line and then charged the approaching Union soldiers. The confederates were all wiped out except for a dog that they had with them. I believe the movie was in B&W.

There's a scene in The Good, The Bad, and the Ugly?