I had to look it up. The Movie was Rio Lobo. The plot was that Wayne, a former Union Colonel finds the Rebs who did this during what I believe was their time of awaiting official parole. Wayne told them what they did was an act of war and he had no hard feelings. However, he knew they had to have a Union accomplice and he wanted to find this traitor.

Wayne meets the Rebs back in Texas and together they fight the traitor (Victor French) who has taken over the local town using a corrupt Sheriff to do his dirty work. Of course the good guys win. The odd thing is that the Rebs and Wayne were both portrayed as decent folk. Imagine that. There was actually a time when people in this country could conceive of those kind of things. I miss the Duke :-(