Westerns are set in the Wild West, which is more or less an exaggerated version of a real region, the western two thirds of the USA, during a real time, usually the 19th century, and mostly in a few decades between about 1860 and 1900.
And because westerns are more or less historical fiction, historical people, places, things, and events are often mentioned in westerns and sometimes depicted.
And how accurately are historical persons, places, things, and events depicted in western movies and television episodes?
There is a blog called Jeff Arnold's West, which has reviews of hundreds of westerns. Arnold often comments that westerns aren't intended to be educational or true. And when he reviews a movie that claims to be a true story he usually comments that the more a movie claims to be true the less true it is likely to be.
For example Walk the Proud Land (1954) opens with this narration:
“The story you’re about to see is true. It happened the way my father told it to me. It started long before I was born, on a hot dusty afternoon in 1874 when he rolled into Tucson on top of a stagecoach.”
The movie is based on the 1936 book Apache Agent by Woodworth Clum, a biography of his father, John Clum (1851-1932), who was the Indian agent at the San Carlos Reservation in Arizona from August 4, 1874 to July 1, 1877.
The events in the movie seem to happen over only a few weeks of fictional time, even though the historical events it is based on happened over 2 years, 7 months, and 28 days from August 4, 1877 to April 1, 1877.
That might be considered a dramatically necessary time compression, but the movie makes big historical changes like making Eskiminzin (c. 1828-1894) the head Apache chief instead of merely the chief of one Apache group, and having a fictional General Wade in charge of the army in Arizona, making Tucson and San Carlos much closer than in real life, etc., etc.
One might expect that John Clum could have told a somewhat garbled account of his experiences with the Apaches to his son Woodworth many years later, but on the other hand Woodworth could have checked the stories he was told with various books about the Indian Wars published before 1932. I suspect that most of the historical inaccuracies in Walk the Proud Land (1954) are the changes that scriptwriters Gil Doud and Jack Sher made to the story.
As Jeff Arnold says:
It was also historically very dubious, normally OK for a Western but if it’s plugged as a biopic and opens with the pronouncement “The story you are about to see is true”, well, it should be better in that regard.
Similarly, White Feather (1955) opens with the narration:
“This is the northern ranges of Wyoming. The year is 1877. What you are about to see really happened. The only difference will be when Indians speak they will speak in our language so you can understand them.”
But every single character in the movie is fictional. Part of the plot involves the Northern Cheyenne moving south to the the Indian Territory in 1877, which is loosely based on historical events, and another part of the plot is based on another historical event that happened 13 years later in 1890, when two young Cheyenne made a date to fight the soldiers and be killed. Naturally the film doesn't give the true names of those 2 young Cheyenne who made an appointment to die, or show one of them as actually being only thirteen years old.
As Jeff Arnold says:
The opening words, spoken by Tanner/Wagner in voiceover, are “What you are about to see actually happened”, a sure sign that we are in for unhistorical melodrama, which is what we get.
As these examples show, when a western claims to be a true story, It is probably at least ninety percent fictional and no more than ten percent historical, and often much, much less than ten percent true.
I think that films set in recent decades that are allegedly based on true stories would probably have a higher ratio of fact to fiction than westerns that claim to be true stories. But a higher ratio of fact to fiction should not be interpreted as being mostly fact with a tiny little bit of fiction thrown in. I suspect that in most cases the ratio might be 20 percent fact to 80 percent fiction, 40 percent fact to 60 percent fiction, 55 percent fact to 45 percent fiction, etc., etc., etc.
So in my opinion if someone watches a movie that claims to be true or based on a true story and is interested the subject of the movie, it would be a good idea to read about that subject to find out how much truth and how much fiction is in the film.
Or go to a website like History vs Hollywood: http://www.historyvshollywood.com/3
Or maybe check if the movie is discussed in "Based on a True Story" trope in TV Tropes: