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 <WSD>miles</WSD> northwest of Prairie du Chien , until then the most advanced United States post . 
Meaning of the word marked with <WSD> tags:  a unit of length equal to 1,760 yards or 5,280 feet; exactly 1609.344 meters