The first source I checked for information was the book, MONTANA PLACE NAMES, FROM ALZADA TO ZORTMAN, which was published by the Montana Historical Society in 2009. I am fascinated in how a name is chosen when a new community springs up. According to this source, the name Sedan was chosen by an early settler, for his former home of Sedan, Kansas.
A quick internet search netted me the following information:
The area was first settled in the mid 1880's and the post office was opened there in 1891. By 1895, Sedan had its first school and by 1898, construction had begun on a Methodist Church. Sedan never became a real town, with houses and such. Instead it was a small hub for the farmers and ranchers who made their homes in the area.This didn't stop them from being a real community, They even built their own telephone system, The East Flathead Telephone Company, in 1905 so that the area farmers could keep in touch. A cheese factory was opened in 1914, creating a market for the dairy farmers. It was in business until 1926. Over the years there were a store, a community hall, and a gold mine. It is hard to imagine, with only one building still standing, that Sedan was a lively place that hosted dances, rodeos, church and 4-H clubs. Now even the post office is gone, the mail coming from Wilsall, but the ranchers who live in the area probably still consider themselves residents of Sedan.