Western
Lonesome Dove is a Western with an identity crisis, just like America was having an identity crisis itself in the late 1800s. Around 1870, there were still only 37 states. A lot of the country was wild territory populated with bandits and grizzly bears.
Larry McMurtry doesn't glamorize the West, although there is some romance and nostalgia in the book. But even in the context of the book, the characters are looking toward the past, too. Many wonder if civilization is headed in the right direction. Bison are disappearing—but so are banditos. Call especially misses when the West was a more volatile place. He liked having to keep an eye out for Mexicans, Indians, and other bandits. To him, civilization is boring.
So Lonesome Dove isn't just a trip to the rodeo. That's one of the reasons why this book takes up so many pages: it's about a lot more than just cows, cowboys, and Indians.