She Wore a Yellow Ribbon (1949) on Netflix

After Custer and the 7th Cavalry are wiped out by Indians, everyone expects the worst. Capt. Nathan Brittles is ordered out on patrol but he's also required to take along Abby Allshard, wife of the Fort's commanding officer, and her niece, the pretty Olivia Dandridge, who are being evacuated for their own safety. Brittles is only a few days away from retirement and Olivia has caught the eye of two of the young officers in the Company, Lt. Flint Cohill and 2nd Lt. Ross Pennell. She's taken to wearing a yellow ribbon in her hair, a sign that she has a beau in the Cavalry, but refuses to say for whom she is wearing it.