By the terms of the 1851 Treaty of Fort Laramie between the United States and seven Indian nations, including the Cheyenne and Arapaho, the US recognized that the Cheyenne and Arapaho held a vast territory encompassing the lands between the North Platte River and Arkansas River and eastward from the Rocky Mountains to western Kansas. This area included present-day southeastern Wyoming, southwestern Nebraska, most of eastern Colorado, and the westernmost portions of Kansas.