Task: Question Answering
Input (English): Passage: The Red River Trails are less well known today than many other pioneer trails and trade routes in North America, and do not occupy as large a place in folklore as the great western trails in the United States and the fur-trading canoe routes of Canada. They were neither fought over nor the locus of battles (with the exception of the Dakota War of 1862), and although hazardous at times, other trails presented greater dangers. It may be that this relative lack of attention is due to the fact they did not lead to annexation of any territory to either of the nations in which the trails were located.The trails nevertheless were instrumental in the development of central North America. Traffic over the West Plains Trail sustained the Selkirk Settlement in its early years. The trails also gave settlers of that colony and their Métis neighbours a route for migration as well as a highway for trade that was not dependent on the Hudson's Bay Company. As usage grew, old fur trading posts became settlements and new communities were established along the cart routes.  The trails pioneered by the fur brigades accelerated development of Minnesota and North Dakota, and facilitated settlement of the Canadian northwest.
The trails had profound political effects during a time of Anglo-American tension. Both Britain and the U.S. were concerned about each other's cross-border influences. Born out of commercial needs and located by the dictates of geography, the trails helped create and contribute to these international influences and the tensions which resulted. The United States sent military expeditions along the route of the trails to assert national interests in the face of the continued British presence in the northwestern fur posts on soil which the U.S. claimed. The Americans were also concerned about the establishment of Lord Selkirk's colony as well as British claims to the Red River Valley. Finally the U.S. wanted to curtail Britain's attempts to get access to the Mississippi, access implicit in the Treaty of Paris ending the American War of Independence, and which Britain sought into the nineteenth century.  The United States' assertion of dominion over its new territories parried and reversed the British domination of the fur trade in the upper Mississippi valley, which had continued for decades after the Revolutionary War settlement which had assigned those territories to the new nation. 
Question: What is the acronym for the United States?
Output (English): U.S.