Rio Lobo (1970)

During the Civil War, the Union Army's gold was being stolen by a band of Confederate soldiers. When the protégé of Colonel Cord McNally is killed during one of their raids, he tracks them down, but is captured. The leader, Captain Pierre Cordona wants the Colonel to lead them away, but instead leads them into their territory and captures Cordona and his Sergeant, Tuscarora. He knows that the only way they could get away with their raids is if they had inside information, which someone from the Union Army gave them. They refuse to tell McNally anything, and are imprisoned. When the War ends, McNally visits them and asks them to tell him from whom they got their information, but unfortunately, they don't know who they are. Tuscarora is on his way to Rio Lobo, Texas where he grew up. McNally then tells them that if they should ever come across them again, to contact him through a friend of his who is a Sheriff in Texas. Later, McNally is contacted by the Sheriff on instructions from Cardona, but when he arrives, Cardona is indisposed, so while they are waiting, a woman arrives and wants to report to the Sheriff a murder that took place in Rio Lobo, but the Sheriff claims that there's nothing he can do. Later, a posse from Rio Lobo arrives and wants to take the woman away, but she claims that the leader is the killer about whom she is talking. Later, a gunfight ensues with Cardona joining in, and he says that the leader is one of the men, for whom McNally is looking. He then tells him that Tuscarora contacted and told him that he saw one of the men for whom the Colonel is looking, who the assumed is the one who was killed, and that there is trouble in Rio Lobo, and he needs help. So McNally, Cordona, and the girl go to Rio Lobo, and they discover all sorts of graft and corruption going on.

IMDb: https://www.imdb.com/title/tt0066301