To begin with, this is a gorgeous looking movie, shot in glorious black & white with beautiful, sweeping landscapes. The acting is also great in Rio Grande; John Wayne is a perfect fit for the role of Col. Kirby Yorke, the gruff and tough yet vulnerable cavalry commander. In addition, he has great chemistry with Maureen O'Hara who plays his estranged wife, Kathleen. The rest of the cast is really good as well: Victor McLaglen, Harry Carey, Jr., Ben Johnson, and Chill Wills to name a few.
Besides being a rousing tale of adventure, the film also deals with some deeper themes such as honour, duty, and self-sacrifice. As well, set about 15 years after the Civil War, the movie gives a glimpse of how profoundly and deeply that event affected the lives of those who experienced it and continued to be a source of bitterness and grief. If you only ever watch one film of Ford's trilogy, it should be this one.
"But he must learn that a man's word to anything, even his own destruction, is his honor."