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 <WSD>United</WSD> States post . 
Meaning of the word marked with <WSD> tags:  North American republic containing 50 states - 48 conterminous states in North America plus Alaska in northwest North America and the Hawaiian Islands in the Pacific Ocean; achieved independence in 1776