Two Mules For Sister Sara (1970)

Set in Mexico, nun Sister Sara (Shirley MacLaine) is rescued from three cowboys by Hogan (Clint Eastwood), who is on his way to do some reconnaissance for a future mission to capture a French fort. The French are chasing Sara, but not for the reasons she tells Hogan, so he decides to help her in return for information about the fort defenses. Inevitably, the two become good friends, but Sara has a secret.

IMDb: https://www.imdb.com/title/tt0065134