 At the beginning of the Civil War, Indian Territory had been essentially reduced to the boundaries of the present-day U.S. state of Oklahoma, and the primary residents of the territory were members of the Five Civilized Tribes or Plains tribes that had been relocated to the western part of the territory on land leased from the Five Civilized Tribes.In 1861, the U.S. abandoned Fort Washita, leaving the Chickasaw and Choctaw Nations defenseless against the Plains tribes.Later the same year, the Confederate States of America signed a Treaty with Choctaws and Chickasaws.