Sentence:  Activity by British traders and the presence of a colony on the Red prompted the United State War Department in 1819 to send Lieutenant-Colonel Henry Leavenworth from Detroit to put a post 300 miles northwest of Prairie du Chien , until then the most advanced United States <WSD>post</WSD> . 
Meaning of the word marked with <WSD> tags:  military installation at which a body of troops is stationed