 Although the United States gained official possession of the region following the conclusion of the American Revolutionary War, British influence on its Native American allies in the region remained strong, especially near Fort Detroit.This influence caused the Northwest Indian War, which began when British-influenced native tribes refused to recognize American authority and were backed in their resistance by British merchants in the area.American military victories in the region and the ratification of the Jay Treaty, which called for British withdrawal from the region's forts, caused a formal evacuation, but the British were not fully expelled from the area until the conclusion of the War of 1812.