The story promotes manifest destiny by taking place in the Southwestern desert during the Apache wars. It's a time where white settlers were pushing into indigenous lands. It's showcased through the stage route, stations and towns like Gap station. It's actively showing American expansion through the frontier. Even though it's a hostile zone, it's treated as a place that is controlled and "travel friendly". The Apache resistance is present, hence the war like tension. The coach was attacked by them to emphasize it.